the result of a process complex enough that it's very difficult to predict the outcome or identify the root cause.

Difficult's a two-place word, and so I'm not sure it makes much sense to argue about whether or not something is 'objectively' difficult, instead of difficult at various states of knowledge.

it carries with it an assumption that disowning gay sons has evolutionary roots, or is ingrained behavior in humans, or is common.

It's not quite that disowning gay sons has evolutionary roots, but that disowning gay sons is not so heavily disfavored as to be extincted. For example, cultures where childbirth is prohibited mostly die out, and so on. But even less obvious things that have an effect on reproductive success are strongly motivating; in cultures with prohibitions against masturbation, those prohibitions are mostly not followed; in cultures where doctors tell mothers to avoid touching their infants because of disease risk, those prohibitions are mostly not followed, and so on. (The impulse for mothers to touch their babies seems very strong, and also very healthy--it actually lowers disease risk by informing the mother what antibodies she needs to produce for her child, and seems critical for proper psychological development.)

And traditional behavior gives us an imperfect window into the economics of the past, which is what's under discussion when we talk about historical selective fitness. If gay sons were helpful enough with nephews and nieces that it was as if they had had their own children, it seems to me they would be welcomed and lauded as examples of loving selflessness. But if gay sons were reproductively disadvantageous, and in particular if it was reasonable to expect that homosexuality is contagious, then there's little cost and some reproductive benefit to forcing them out of the home.

(I should note that the hypothesis that one gene causes both female fecundity and male homosexuality is also consistent with disowning gay sons, but I think that one has other challenges.)

After all, my family didn't disown me, and it seems like disowning gay sons is becoming increasingly uncommon.

Thankfully, people are much more motivated today by individual and relationship satisfaction, neither of which disowning is helpful with. (My family didn't disown me either.)

Does it make sense for families to disown children for being the wrong religion?

It suspect it made sense for religions to disown members that fail to disown their children for being the wrong religion.

Does it make sense for families to disown children for being pregnant?

It looks like control over sexuality was a big deal, and as a first-order effect it seems that signals of that control would heavily impact someone's price on the sexual marketplace. As a second-order effect, it seems that the harsher penalties are for not being controlled, the more likely people are to submit to control. But in less status-stratified societies? Probably not--and it seems like this is mostly a class thing in the societies that I'm familiar with.

This is mostly my speculation, though--I haven't read much on evolutionary accounts of how parents should respond to teenage pregnancy in various environments. I expect someone has thought about this problem.

The explanation is the same as most other "repulsions", that you happened to develop a response of visceral disgust to a particular stimulus.

"Happened" is typically used when a result is undetermined or not strongly expected: "the die happened to come up a six." It would sound weird to use it when a result is determined in advance: "the cup happened to fall once I let it go."

I've seen nothing to show me that revulsion to women is somehow inherent to homosexuality, while I'm not asking you to prove it, I hope you have a good reason for believing it.

It's certainly not inherent to all homosexuality. Whenever this topic comes up, this part of American on Purpose comes to mind:

I also remember feeling physically ill at the sensation of the bristle on his chin against mine and I'm still haunted and creeped out by that. My gay male friends tell me that they have felt the same about experimental physical encounters with women.

(I don't seem to have this reaction, myself. But my experiences are also minimal.)

Do they?

Does who what? Do families disown gay sons? Yes; my first boyfriend was kicked out by his family, and it seems a disproportionately large fraction of homeless youth are LGBT, with some large fraction of those claiming their parents forced them out.

This is true, but it's not the same as in bees: there, it is in the interest of each worker not to have children and raise its sisters instead.

Is that really in need of an explanation, though? Some people are repulsed (by stuff), and others aren't.

It seems to me that any mental feature that can be explained should be explained. It makes great sense to me why there are people who feel visceral disgust at the prospect of sexual activity with their siblings, and that there are men who feel visceral disgust at the prospect of sexual activity with men. (Interestingly enough, this sort of 'homophobia' seems more heritable than homosexuality is, which makes sense--this is actually selectively positive!) So what's going on with a man who feels visceral disgust at the prospect of sexual activity with women? Is it the same sort of homophobia reflex, but miswired? Is it something else?

If you don't find explaining things interesting, well, I recommend a career in something other than science :P

Not every individual in a species has to be geared for reproduction

How does evolution work? There are two core pieces: variation, and selection. Variation is the uninteresting bit, and selection is the interesting bit: entities that reproduce themselves become more common in later generations, because they reproduced themselves.

It's not a question of what "has" to happen. It's a question of "what happens." Similarly, explanations need to fit together with every other part of what we see. Suppose a model where gay uncles help people in their family raise children, are generous with their wealth to family members, and so on. In such a case, does it make sense for families to disown gay sons?

(Remember, we're trying to explain why 3% of men are gay. We can't accept any explanations that would make, say, half of men gay, because that doesn't fit with the facts!)

And considering that, it might be the case that the "gay uncle" really does help the reproductive fitness of the group, or at least, doesn't hurt it so much that genes that create the potential for homosexuality are selected against.

Are you explicitly suggesting group selection? It doesn't work outside of very special circumstances.

(The linked wiki article comes across as rather unpleasant in tone. Your user account is new and if you're new to the site, I don't want you to make you think this is typical. In that case please just use the wiki page as a link index to other articles about the subject, and for a counterpoint on when group selection might work after all, see e.g. this post.)

In bees, for example, the vast majority of bees are totally infertile with just a handful of fertile bees in a single hive.

Eusocial insects make it work by making workers more related to their sisters than they would be to their own daughters. So workers prefer to raise their sisters than to give birth to their own sisters.

Part of the reason is probably because of a conflict between the mother, father, and baby. The baby gets all of the benefit of, say, having a bigger head at birth but some of the cost is born by the mother and little by the father so from the baby and father's viewpoint the head size will be bigger than it would be for the mom.

But they don't. Anyhow, that's still a 25% fitness cost, which is huge.

On the other hand, 'obligate' is a strong word to use for things as complicated as human behavior.

Specifically, there are gay men who are repulsed by the prospect of sex with women, and then gay men who are simply not interested (or not as interested) in sex with women.* It seems to me that 'obligate' is meant to refer to the first cluster, and should have a reproductive penalty roughly the same as male asexuality. (Figuring out what percentage of the population falls into that cluster is hard, since you can't really go off self-reports of preference; you want self-reports of anti-preference.)

*Straight men can consider how they would feel about having sex with another man; for some, there's a visceral disgust reaction, and for some there's just a "but... why would I do that?" reaction or a "eh, any port in a storm" reaction.

Knowing several older male homsexuals with biological children via socially-imposed marriage customs, I don't think the effect is as large as many assume under many environments.

In farming societies where monogamy is the norm and marriages are economic arrangements, it seems to me that the reproductive cost of sexual interest in men is minor (or possibly positive, if men in power are willing to trade resources for sexual favors).

But in societies where polygamy is the norm and men compete for women, it seems likely to me that any man who is less interested in winning is less likely to win, and the costs of sexual interest in men might grow significantly.