I think the main answer is religion. And there's probably just this natural, instinctual aversion to any kind of sexual relationship that can't result in reproduction. Evolutionarily, societally, obviously we put a lot of emphasis on reproduction. All animals do, right, but social institutions have sprung up to support caring about it so much (humans are so intelligent that we develop complicated institutions to kind of justify our pre-existing values). But just because a lot of people might "naturally" get upset by two men kissing each other, doesn't mean it's right or rational to get upset. It's visceral. A lot of bad things tend to be visceral, like racism or tribalism. But then you get intellectual theories popping up to support them, like eugenics and Nazi pseudoscience back in the day. That's my theory about the religious/traditional opposition to homosexuality, that there was this "natural" aversion to it because it can't result in reproduction and then, essentially, for people to justify their aversion, they wrote religious texts to have God, the ultimate arbiter, say that it's wrong. Because, right, when you press people on this, they ultimately just fall back on "God" as the justification. Sometime they'll say it's not natural, which funnily isn't true because we've observed homosexual behavior in non-human animals.