The notion that it is impolite to discuss sex, religion, or politics at a dinner party is generally considered old-fashioned these days. In fact, it's my observation that people talk about nothing else! (There's a joke somewhere in here about the Republican primary but I'll refrain.) But is there any topic you would forbid, or at least discourage, at the dinner table?
Of course much depends on the company you're keeping. In many households, the talk around the family table might sound different than what comes up while sipping a post-dinner brandy with your friends. And how well you know those friends also makes a big difference. A heated political discussion or a bordering-on-TMI sex discussion might feel more comfortable and therefore more apt to happen when you're with old friends and people you know well. Certain friends, too, will have an impact depending on their inclination to stir up controversy or their allegiance to a specific point of view.
Work colleagues is another category that has an influence on dinner topics. How well we know them, how much power they have over us at the office, and the kind of environment we work together in will all influence the tenor of the evening. For some people 'mixed company' (usually meaning children or people of the opposite sex) will also dictate the topics of conversation.
Sometimes it all depends on what we're in the mood for. A lively and controversial discussion can be a lot of fun and leave everyone energized and with food for thought. And at other times, a quiet, more mellow evening is more appropriate.
Is there a topic you deem too impolite to discuss at dinner? Do you ever invite friends over that you know will clash, just for the liveliness of it? Have you ever forbidden a topic at the dinner table or stopped a controversial conversation in mid-point because it was going too far?
Related: Entertaining: Do You Give Your Guests Assigned Seating?
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Bacsac Bacsquare 04...

Anything health related. I have an incredibly awkward boss who discusses his knee replacement surgery whenever possible. Including, recently, at a convocation dinner for faculty. And trust me, he leaves no details out. Thus ruining everyone's appetite.
Oh heck, I grew up with a family who discussed politics constantly. There would be uproarious laughter about right-wingers and Catholics (even from Auntie and Grandma who were Catholics!) The only thing I frown on are conversations where people bicker, or (even worse) graphic descriptions of someone's body or sex life. That should be kept for the diary.
Personally, I don't feel like any topic should be off limits at the table--so long as you are in the home. If you are in a restaurant it is common courtesy to keep extremely controversial topics, which are unique to any group of friends, off the table. Living in DC my whole life I have found politics do not come up particularly often, as no one wants to talk shop, but sex...that topic could go on all night.
That being said, having worked as a bartender, I think the "no sex, religion, or politics" rule is more appropriate to a bar scene. I've seen grown men in suits and ties throw down over politics, same goes for more than a few ladies and various sexual topics. If you don't know the people and the alcohol is flowing, try to steer clear of things like "I know you're kosher, but bacon is God's gift to man." Because we really don't enjoy bloodied patrons as much as you think, it usually winds up with your bartender having to comp the whole bar a round after the cops break up the fight. Don't be "that guy."
Never talk about other food. Talk about the food you're eating but never about other food you've had. That's the only rule we've had at the table.
I was actually just discussing this last week (http://ourordinariness.wordpress.com/2012/03/15/dumbing-down-dinner-or-why-i-hate-small-talk/). I'm pretty sympathetic to keeping discussions of personal finances to a minimum (the economy/politics I like though). I hate to have guests who don't like meaningful topics; good food needs good conversation, and I hate to feel like I've wated a dinner party because all we talked about was the weather.
i never thought about rules for dinner party conversations. the more wine involved the less rules we'd follow if there were any to begin with! i just try to change the conversation when i sense any discomfort. one time i was at a dinner party where the conversation turned to the various toilet designs of other countries!
Food restrictions/allergies. Even though I'm vegetarian, I hate being asked why as we're eating, and similarly when people have food allergies and folks pester them about what they can or can't eat or the graphic descriptions of what happens when someone with an allergy eats said item.
Anything in the spirit of debate (persuasive or otherwise) rather than conversation is generally a bad idea. I came to enjoy a meal and light conversation, not have someone try to change or simply insult my views on food, politics, movies or whatever.
When I first started seeing my boyfriend, he was horrified at the graphic descriptions of tummy distress that he'd regularly get at my parents' dinner table (I used to joke that no dinner there was complete without the "GI update"). So I started trying to rein that in when I went over there, with mixed success.
I once went out with some people for dinner at a restaurant for my mother's birthday. One of the guests started very loudly haranguing us with his idiosyncratic political views. We all kept trying to change the subject or joke him out of his mood, but no luck--he was just determined to shout everybody down and go on with his lecture. Yikes. Luckily, my mother said she didn't mind it; I was mortified and never went to an event with that guy again. Talking politics is fine with the right group, but doing it that aggressively is definitely a no-no.
I think it's just good to try not to offend people you are eating with. So, if it's an old group of friends, you ought to be able to discuss anything. But, it may be wise to avoid sex, religion, and politics if you don't know the views of the people you are with (or how passionate they are about those views.)
I agree with the first commenter--health issues. I can handle varying amount of other "off-limits" topics, but seriously, who wants to hear about unappetizing subjects during a lovely meal?
Just yesterday, I was seated between two of my husband's distant relatives at a banquet. I'll spare you the details, but the term "lady parts" was used multiple times by my dining companions. And not in a fun way.
I do not agree with discussing finances at dinner, whether it is at a restaurant or at home. This is true especially when family/friends are around. It makes everyone feel awkward and not want to be there. The same goes for family problems.
One of my pet peeves is having dinner with a group of friends or family and then the fighting couple shows up and puts everyone in a weird position. Like what should we talk about, should I talk to one and not the other, why are they fighting, etc. The bad moods trickle down to the everyone else at the table like a virus. Just go home and stay home.
quinnykins, that's probably the only rule I would enforce too. Nothing makes your food taste worse than thinking/hearing about something else.
As others have said, "controversial" topics when brought up politely and in a non-antagonistic way are fine. But I don't like when people talk about distressing or sad things while we're eating. It ruins the taste of the food. I was once sitting at the dinner table with my dad about to dig into some delicious food and he brought up my dead little sister. We ate the whole meal in silence and I didn't taste anything I ate.
My DFIL is a physician. Since I live near my in-laws I socialize with them a great deal (and love them both to death, BTW.)
I was taken aback by very frank and detailed discussions of their work. But I quickly realized that most folks talk about their work, so now I can listen to their conversations without blinking an eye!
The only iron-clad rule that springs to mind: I don't want to talk about weight loss, weight gain, or dietary guilt at the table. If you're a guest and have dietary restrictions, that's fine; let's talk about them (ideally in advance) and I'll make sure to prepare a variety of things tailored to those restrictions. If you have questions about the dishes, I'll love to answer them! I know dining out can be difficult for people with dietary restrictions (self-imposed or externally imposed) and I want to make it easy for guests.
And the same is true for guests whose restrictions come from a desire to gain or lose weight: I'll cheerfully tell them the ingredients and cooking methods so they can make their own choices. But if you're going to sit at the table and talk about how you need to gain or lose weight, or how you have already gained or lost weight, or (heaven help you) about how I should gain or lose weight, I'm going to politely shut you down. Every. Single. Time.
I think this rule is more about not asking intrusive questions or foisting your opinion on a captive audience. A mature adult should be able to talk about topics like finance without asking invasive questions, sharing too much information, or giving a lecture. For example:
Appropriate: "I just read that women's salary growth is exponentially higher than men's."
Inappropriate: "How much does your job pay? I'm on track for a huge raise."
Sadly, I think there's a plague today of adults who can't respectfully discuss a hot-button topic. When people can't separate their emotions from their opinions, a friendly discussion isn't possible. This isn't limited to the big three (sex, religion, and politics) -- people can get aggressive about child rearing, diet, spending habits, schools, sports, or even where you shop.
I've had perfectly nice evenings tainted by arguing about football all night. On the other hand, I've had boys ask for my number after we'd argued about football all night, so, you know.... it depends.
Anything unsavory or confrontational. I once had to sit through an entire dinner with my in-laws where they only discussed different words for "panties" and toilet humor. That was one of the better dinner discussions we had. They normally just involve being interrogated nonstop, often regarding private matters.
Then theres always the inevitable when theres a solider or veteran present of somebody getting extremely confrontational about them being government pawns and baby killers. Fun times.
We constantly talk politics at home, and I grew up on that, but we zip it at parties/in public unless it comes up. I'll have the conversation, but I won't start it.
Health problems are a total no thank you, and I am in hearty agreement with the poster who asked for no discussion of dietary restrictions. I am a vegetarian and whether you're honestly curious or out to pick a fight, really, I don't want I am and am not eating to become the focus of conversation. I feel it puts the hosts on the spot, because either they made me something special or I'm not eating something they made, and it's just rude to THEM to bring it up and make an issue of it.
We had some guests over a while ago who were new to the area, and the female half of the couple talked ALL NIGHT LONG about what a terrible place this is to live, how much better the place was that she moved from, etc., etc. I tried to tell her about some of the good things here--okay, it's not a metropolitan area, but the economy is pretty strong, there are lots of outdoorsy things to do, etc.--and she just talked over me and kept going. Luckily, everybody else seemed to take her rudeness as amusing and it didn't ruin the meal. Man, that's a big DON'T right there.
Just last night as we were eating dinner my darling boyfriend started talking about something which involved the use of the words "pig shit" and I butt-in immediately with, "really, honey? Eating."
Health problems, dietary restrictions, and food guilt and definitely big appetite killers for me.
Though we did attend a nice sit down dinner a few weekends ago where we all talked about IUD's and women's health for most of the evening, because that's what most of the people there do for work, and it didn't feel inappropriate at all.
I shouldn't have to make rules about conversation topics. Most topics are fine, it is the people who are discussing them who wind up ruining them. If you can't keep from being aggressive and rude, or are oblivious to the fact that what you are saying is making people around you uncomfortable and ruining a conversation, you lack the requisite social skills to attend these kinds of gatherings and you won't be invited back. It is not hard to tell when what you are saying is not going over well (whether awkward silence/looks, or tempers flaring), and it should be very clear when it is appropriate to drop a topic and move on. I feel like I shouldn't have to set rules for the table; if I have a guest who is unable to navigate a conversation without seeming like an ass and bringing others down, then I will apologize privately to other guests afterwards for making such a bad choice in having them, and leave them out of future plans.
Seconded on barring discussion of personal finances good or bad, which I have seen ruin a party. We want to hear about what you're doing at your interesting new job. We don't want to hear how much more money you're making or the pay cut you took.
Food shaming is a big no-no for me as well. I have regular Feminist Brunch with a bunch of women friends and one of them has sort of made it her personal mission to interrupt that kind of talk, which we get so accustomed to that even in "feminist" circles you hear all the time "I was bad yesterday and had second helpings of dessert" or whatever. Now the first time somebody does it, Danielle (kindly) tells them that they can eat or not eat whatever they like for whatever reasons they want, but we're not going to assess our personal and moral worth based on what we do or don't eat.
Anything to do with bugs, insects, creepy crawlies.... eeeeww
Anything that might be stomach turning. Several people gave examples above.
Anything involving why someone is not eating all of the food options as the table. As someone else mentioned, food allergies or ethical food preferences are not something that should be brought up.
Obviously, it depends on the context -- who you're eating with, where, what's the occasion, etc. But generally speaking, I'd say don't discuss anything gross (ie, bodily functions), overly personal (ie, sex life) or buzzkilling (ie, dietary restrictions).
The only restriction is when you suddenly find that you've been the only one talking for 10 or more minutes...probably a good indication to let the conversation flow more than the wine! I have no topic I can't bear to hear, I'd only ask that at my table every conversation only lasts until it has run its course.
I don't want to hear about children's toilet habits (surprising how many parents think this is an OK topic of conversation), and I don't like any form of one-up-manship. Anything else is OK, as long as it's lighthearted and nobody is made to feel defensive.
Come to think of it I agree with posters who say it depends on the company. I know a few medical and veterinary folks, we have a great time talking shop while eating. Heck, one of my coworkers used to eat lunch while watching surgeries through the hall window! So it depends, just make 100 percent sure your friends are on the same page, and don't hog the floor.
I'm a microbiologist working in a children's hospital and I often tend to forget my audience and discuss bacteria, fecal cultures, etc. I've learned to tone it down a bit, but the truth of the matter is that this is my job and at meal times at work we routinely discuss these topics without a second thought. Most of my friends and family have gotten used to it but I'm much more careful around strangers and friends' new beaus. My husband used to tease me that a meal with me wasn't complete without the mention of the "P-word", his euphemism for placenta.