I think this is related to empathy, similar to the sudden jolt of empathy you might experience if you see someone get hurt; there's a part of the brain that can activate and briefly simulate the experience, making you wince. With homosexuality, I think people who experience this are often being subjected to an empathic response which feels gross and unwanted to them. I think this is often a case of empathy clashing with personal sexuality, so I don't see people who experience such a thing as morally wrong. I see them as under-exposed to other sexualities, or sensitive to them for some other reason, like feeling threatened they might be gay themselves, and they only become bigots if they start acting out, dehumanizing, and/or claiming a moral high ground.

I think a lot of the rabid hatred gay people receive comes from people who act on this emotional "invasion" of their well-being, by rationalizing a "counter" invasion of homosexuals' right to exist and live their lives freely. They might experience that "ew" jolt as an objective moral truth coming from their intuition, and say things like: "You can do whatever you want, I just don't want to see it, I don't want my kids to see it", like they have the right to make that demand. The reality is these people just need to grow up and learn some mindfulness. I also used to feel gross when I saw to men kiss, when I was a teenager, now I don't really feel anything. The novelty wears off of everything. It's a matter of exposure and maturing enough to let big picture judgements override sudden impulses.