What causes homosexuality?

One might ask for what just is it that determines our "sexual orientation?" Most of the details of how to have sex and who we think beautiful are learned, as are fetishes, and all that comes through life.  It is probably true that almost all men (gay and straight) are born with an inherent desire for youthful partners who are of optimal breeding age, and it is probably true that most women have a different, more "will they be loyal and help me with my baby?" set of priorities.  Otherwise the details get fixed by experiences of what is pleasurable and what isn't.

The sexual orientation also has to be kept separate in one's mind from one's sexual identity.  I'm gay but I know and like the fact that I'm a man and have no desire to be a woman, either physically or in bed.  That is a separate set of issues.

Now what is it that makes for orientation?  I think it is "turn on" buttons, and there are two sets of these.  One is an attraction to smooth skin, broad hips, breasts, female genitals.  The other is an attraction to rougher skin, narrow hips and broad shoulders, male genitals.  Except for bisexuals, who appear to be heterogeneous here (if it's genetic), we get one or the other and it is in place from the beginning, although of course it doesn't really present itself until the puberty hormones set in.

There does seem to be pretty strong twin evidence of an inherited component (identical twins are more likely the same orientation that fraternal twins), but there could be a uterine component too (there is a slight prevalence of same orientation in fraternal twins).  Brothers raised together and brothers raised separately show no particular concurrence, so we conclude it is not overbearing mothers and weak fathers.

What one inherits seems to be a genetic "turn on" pattern, in humans mainly visually oriented.

Of course one could ask what is the survival value of having the patterns inverted occasionally.  I don't think there needs to be one (although theories abound).  It could be just something that evolution has never had any reason to evolve prevention mechanisms for.   In most societies when a boy is gay, the brother steps in on the sly, and the family name goes on.  There is not that much of a selective disadvantage, and it can be offset by the additional help raising the babies.  Besides, among hunter-gatherers, it could be useful to have members of the tribe who the hunters know will not cuckold them remain behind with the women to provide better security.