Bisexual means they have sex with males and females. That is all it means. So yes, bisexual men do exist. No clue how this is a conversation piece at all when the word itself is self-explanatory.
They are a part of the Rainbow Flag coalition, and have their letter designation included prominently in the alphabet lineup. If they did not exist, no space would need to be made for them in that community's designation list to recognize their participation.