Line up and I'll hand out the facepalm images. Don't worry, we have enough for everyone.
We should probably have a rigorous and shared definition of bisexuality before answering this question. That said, I think sexual orientation is a combination of both genetic and environmental factors. If Zevran were born and raised in a completely different environment, it's possible he wouldn't be bisexual.
That's fair enough. It's part of why I am inquiring about it, because I have read up on the idea of biological and environmental factors influencing sexuality. The idea of the fluidity of one's orientation and so forth, and what may (or may not) influence those things.
It may be, but I don't think it ought to be a consequence of his lifestyle though, so when assessing things such as the believability of a character I was hoping on examining the notions of "This person is believable because of X," rather than "This person is believable, and happens to have X."
For what it's worth, Inquisitor Julianos, I'd be lying if I said that I couldn't relate first hand to your perspective. It's something that I don't really believe so much anymore, but I had some people poke me and make me go "must this be the way things are" on a variety of topics, including sexuality.