[user104658]

People are complex and it can be either. It's been fairly conclusively proven that many (most, I think) homosexuals are literally "wired differently" from birth. However, the psychology of sexuality is very complex and experience - especially early experiences - can affect adult sexuality.

I also personally believe that some degree of bisexuality is much more common than many people care to admit, with it being socially "drummed out" of most people at an early stage.