It is simple. There are physiological differences between homosexuals and heterosexuals, number man, in how they respond to varying stimulus as well as the activity in brain activity and formation.
Hell, the fact that homosexuality is found in other primates show that it is a biological function. If you are a Darwinist you would realize that homosexuality is evolutionary maladaptive and that would be correct, but it doesn't change the fact that it is largely biological.
Links on a few studies:
http://www.pnas.org/cgi/content/full/95/5/...ourcetype=HWCIT
http://www.pnas.org/cgi/reprint/89/15/7199...ourcetype=HWCIT
http://www.pnas.org/cgi/content/full/102/2...ourcetype=HWCIT