To imply that homosexuality is genetic is to be in some degree of ignorance of what should be a rather obvious fact:

Gay folks aren't exactly multiplying like bunnies. In fact, they don't seem to be able to reproduce with one another at all.

Admittedly, some gay people do have kids (obviously with those of the opposite sex), and some people have kids before realizing they're gay. The fact of the matter, though, is that if there were a "gay gene" floating around, homosexuality would probably not be so common as it is today, as being unlikely or psychologically unable to mate with the opposite sex does not exactly predispose an organism toward evolutionary success.

What I think is incredibly likely to be the cause of some homosexuality is a tendency, genetically, (in males, in this case) toward psychological and physical features inclined more towards femininity, psychological introversion, and/or perhaps sexual submissiveness. This may explain why you often see large concentrations of homosexuality in one family or another, while you may see very few in a different family tree.

However, I think it highly unlikely that all or even most cases of homosexuality are entirely the result of predisposition at birth. As is the case with virtually every aspect of the human mind, a combination of genetic and environmental factors mold the sexual psyche, and so lead one person to be adamantly heterosexual and another to be entirely unable to bring themselves to copulate with someone of another sex (an extreme, but a not-so-uncommon one).

Many gender psychologists have gone so far as to speculate that humans, as primarily cerebrally controlled animals, are "born" bisexual (though Freudians and others say that up until five or six they are entirely asexual, psychologically), and that sexual disposition is later determined by socioenvironmental factors and the biological and psychological habits that have been developed over the individual's life span to-date.

I might be led to wonder whether the fact that 2/3 of WolfDaddy's birth siblings are gay or lesbian has something to do with the probable damage to a sense of personal identity and sense of belonging that probably occurs during the childhood of adopted children. I would be incredibly curious to find statistics pertaining to the tendency toward homosexuality in adopted children.

Post script: I am bisexual, if anyone is curious or thinks that my statements were too frank or were in some way offensive to the gay community. I am not a huge advocate of gay pride, or "anything pride" for that matter--what you are or choose to be is what you are or choose to be, and value judgements concerning the same are unnecessary and superficial.
Update, 7/13/02: Tlogmer says re I'm gay, but I'm not sure it's genetic. : There's a possibility you seemed to be unaware of -- there could be several genes that in isolation all do positive things but when present together greatly increase the chance of homosexuality -- this is how disorders like sickle-cell anemia occur (not so say homosexuality is a disorder)

From a conversational, evolutionary standpoint, it is quite a disorder, I think, and that's an interesting point to make. I hadn't thought of it, but it's entirely sound.