I agree with Lingerielady and it still puzzles me that people are still asking questions such as the ones in the original post.
I just don't get it. Some people are bisexual, get over it!
As for the people you know who declared themselves "bisexual", well maybe they are bisexual, maybe they're not.
One thing's for sure: people often lie. It's what makes us human!
So perhaps, your friends or acquaintances weren't exactly being completely truthful when they left their respective partners of the female and legally married sex?
People have been known to lie during such times as divorces as well as throughout whole relationships. It's very rare, I guess, but it does happen....
EDIT: Just in case it is not completely fucking obvious, there's more than a teeny bitsy bit of sarcasm in parts of this post!