I always think that gay people who deny that someone could be bisexual are no different than those straight people who deny that anybody could be 'really' gay. The same goes for BDSM people who don't believe that someone could be a switch. It comes down to a lack of imagination about how other people are not political banners, but are individuals. I say that as someone who is fully straight and has never had any submissive/masochistic feelings (that I can think of).

As I've gotten older, I've become more comfortable with my sexuality, and part of that maturity is becoming comfortable with other people's sexuality as well. You can be the biggest dominant in the world, or the subbiest sub, and that doesn't make a bit of difference to the sexuality of the guy sitting next to you on the train, or the girl behind the counter at the supermarket. I don't care if you've been in the 'scene' for fifty years - you don't have any more right to define another person's sexuality than their parents do or their priest does.