I'm not sure hunter-gatherers typically avoid pregnancy - they're much more free-lovey and screw-it-let's-just-kill-excess-kids-ey than low-tech farmers.

Sex aversion in cultures I'm most familiar with seems to have to do with proof of paternity (individual, small set of individuals, or even just the local band) than with pregnancy avoidance. Sex when you have too many kids to raise is stupid but not gross; sex out of wedlock when you could totally raise a kid is. Don't know if it's because many cultures have incentives to condition for that or if it's innate.

But that's not quite my question. What I'm asking about is why physical and moral disgust have so much overlap. Touching poop then eating is gross, but I don't feel it's morally repugnant. Killing your neighbor is evil, but not gross. So why does disgust leak into morality? I don't think we ever do in/outgroup or fairness or harm/care without a moral element. Most emotions (joy and curiosity and the like) affect moral judgement, but they're not fundamental bases.

And why do we have such specific emotions for the sacred? It's a weird-ass intersection of cleanliness, morality, ingroup bonding mechanisms, appeasing the high-status, and aesthetic appreciation. Who ordered that?

I'm not sure why disgust can be conditioned at all, but we can do that for all emotions anyway and cultures that learn win.

Or interacial kisses?

No, that one's easy. The proper place for a person is among their race, leaking out is matter out of place - impurity, dirt. Plus, whites are better than blacks, so mixing black with white is disgusting corruption, like mixing dirt with food.

Whereas I'd expect basically the Ancient Greek stance on homosexuality: doing men is More Purer, and men are better than women so they're nobler in the sack. (And two women can't have sex, silly.)

Short answer: conditioning.

I stand by this answer.

I used to find gay man-man kissing (or any form of intimate touching between males, really) very gross despite a very strong conscious understanding and notion that it was just as "right" for them as between a man and a woman.

Then, as I noticed and saw more of it, it got normal.

Now I don't find any of it the least bit gross or off-putting anymore, except in rare cases that evoke specific memories.

The just-so hindsight explanation that makes the most sense is that I believed-by-default everything I was told as a child about such things being "bad", "gross" and "disgusting" or even outright "evil" by my peers. However, that's only the slightly-more-likely out of many possible explanations, and I don't have real data.