Sent by LindseyEditor: Mangoes are really only bitter when they're under-ripe, so my guess is that even though they were soft, they still weren't quite ripe yet. The mango should give a little when you press it near the stem, feel heavy for its size, and smell sweet and floral. You might also try a different variety of mango, like Ataulfo mangoes. Baking the mango in something like muffins or cooking it into a sauce would also help reduce bitterness.
As for your poor batch of frozen yogurt, I'm afraid I'm at a loss!
Readers, have you ever had this problem with mangoes? What do you suggest?
Related: How to Cut a Mango
(Image: Emma Christensen)