I think, with some bisexual men... they are seeking out the opportunity to do something they can't do with a woman. Being allowed to be a bottom gives the man a chance to experience something that a woman can't do (without a dildo).
At the same time, some bisexual men love the experience of penetration, giving or receiving.
I'm just offering what I've seen. I'm probably wrong. :wink:
I like the versatility that bisexuality offers a person.