Twin studies- twins raised apart- support biological factors also. No theres no hard proof as such but there is a growing body of evidence in support of differences in biological make-up of gay people. There are no common experiential factors in the upbringing/life of gay people.
And intuition and experience tells me that those feelings/preferences are entirely natural, its biology/chemistry n all that JazZ
...and aswell as the Swedish study (Tom mentioned), Le Vay 1993,
http://blog.wired.com/wiredscience/2...ists-link.html
And I think this might be what Arista is thinking about, something along those lines maybe...
http://www.newscientist.com/article/...explained.html
http://news.bbc.co.uk/1/hi/health/4215427.stm