There is a HUGE basis for homosexuality/bisexuality in the scientific literature. Anyone with a fucking ounce of patience can go find it on pubmed or any other scientific literature search. If you want stuff geared more towards a lay audience, then read Sperm Wars: The Science of Sex by Robin Baker, and read more about evolutionary psychology, namely David Buss, Leda Cosmides, and maybe even a bit of Steven Pinker. Shit, this article just published recently has plenty of studies to support its suppositions:
http://www.sciam.com/article.cfm?id=bisexual-species
http://www.sciam.com/article.cfm?id=bisexual-species