There are all manner of serious theories, ranging from purely psychological (weak father, domineering mother) to completely biological. One strange theory recently advanced (by Cochran and Ewald) is that there is an as yet unidentified virus that causes unborn male fetuses to become gay. An even stranger theory is that of Dr. Ian Stevenson (University of Virginia), who believes that homosexuals are men who were women in a previous life (Stevenson has spent his career amassing intriguing evidence for reincarnation).
Various theories look good from some angles, but always it seems that there is a problem with the theory. One theory popular with some conservatives is that homosexuals recruit, or similarly, that sexual abuse causes homosexuality. While there are plenty of gay men with these unfortunate experiences, which makes the theory look good superficially, it turns out that only a minority of gay men have been sexually abused -- and that lots of men who were sexually abused by men as children grow up to be heterosexual. One clue: most gay men report not being stimulated visually by women (e.g., by erotica), even if they do respond to physical sensation and closeness with a woman. Another clue: most boys identified as "sissies" by other children grow up to be gay. (Perhaps being a sissy causes homosexuality, or being picked on by peers -- or perhaps sissy-ness is actually an early manifestation of homosexuality.)
The important thing to remember is that for most gay men, they do not remember "becoming" gay -- they've always felt they were gay, since their first sexual feelings manifested in early pubescence. Recent research suggests that some highly motivated homosexual men can at least partially change their orientation, but it is still clear that most homosexual men are not able to "change" for long. I strongly suspect that there are actually several different causes of homosexuality, and that in certain kinds of homosexuality, change is possible (perhaps because some gay men are really weakly bisexual).
I for one now believe that acceptance of homosexuals should not depend on some possible biological cause of homosexuality. I believe conservatives should concentrate on encouraging responsible behavior among homosexuals (caring, committed permanent monogamous relationships), but of course I respect those who differ with me on this point.
And let's all sing Kumbaya.
Wake up. I use to think that way about twenty years ago until I started to see the gay movement totally intolerant of my beliefs. I started to see gay activist attacking my religion because we thought homosexuality was a sin, even though we felt that they were free to do what they want. I read articles by gay activist demanding that no one should be legally able to teach their kids that homosexuality is a sin. I've seen our tolerance feed their intolerance.
When I was a child I noticed that some people feared that if you give an inch your opposition would take a mile. At the time I thought how silly and now I understand why.
We have reached the point that we have tyranny by the minority, while the founding fathers worried about tyranny by the majority. Guess they never fathomed how fat, lazy and sassy we would become and that our apathy would ever run this deep.
How about putting your penis in the wrong place?
Seems pretty simple, actually... considering mammalian anatomy...