Actually, while not everyone is bisexual, Kinsey established pretty decisively that almost everyone has bisexual tendencies to some degree or another. It varies, from person to person, with some people (not most!) have virtually no attraction to the same sex, and some people having about 50/50 attraction to either sex. (For the people who are bisexual - that have sex with either gender from time to time, calling it equal attraction isn't quite right - from what I've been able to ascertain from talking to folks, it is more that they are attracted to the PERSON first, and sexual characteristics as a secondary thing, so they could be attracted to a male or a female who is attractive to them in some other way.)
Of course, having such tendencies doesn't make one bisexual in the sense of having sex with persons of both genders. (We'll leave out the notion of non-binary gender here for pedagogical reasons - it just complicates the matter further.)
What I'm saying is that lots of males, otherwise heterosexual identifying males, will get aroused under the right circumstances from seeing another male in a sexual situation. (Same goes for females.) That doesn't mean much in terms of how he identifies in terms of his sexuality. It's just how it is.
Society, in general, is uncomfortable with this fact, although it is meticulously documented and extremely well established.
edit: Just for the record, I'm kind of sorry to report that I appear to be really straight. Having talked to a lot of bisexual people on this forum, I think bisexual people have a lot of love in their hearts, and a lot of ability to accept people for who they are, rather than how they look. I think those are really admirable qualities, and I'm a little sad that I don't share them, I fear. I think this is very misunderstood in our society, and now that I know a little more, it makes me sad.





