Projective disgust is what this is called. It's partially about one's own purity, and it's partially about the dirtiness of the "other". The "other" in the case of many heterosexuals is homosexuality. For many homosexuals it's heterosexuality. And what it means is basically, you're seeing things from the context of a gay person, if you're straight, and see two men kissing. And for straight guys especially, the first thing that triggers them is, when thinking things in the context of a gay person, when in a gay environment, is anal sex. Male anal sex. Secondly, it's male oral sex. It's about the sharing of saliva when two people kiss. It's about a guy going out of his traditional gender role and doing what a woman is *supposed* to do. It's about you being in a situation that you as an individual are insecure with. That you find contaminates yourself.
These attitudes are 100% shaped by social norms. In many countries, men kiss as a greeting. In my life, I live with a lot of social liberal type straights and gays alike, alternative types. I kiss my straight friends, cuddle with them even. It's not a big deal at all, we're all very affectionate. It depends on how you have been raised. It depends on if you've been indoctrinated by a homophobic society to hold a homophobic mentality.
Disgust is a moral based trait that becomes physical. The kissing itself isn't what disgusts you. It's the fact a MAN is doing what he isn't "supposed" to do that you find disgusting for the reasons said above. Straight men don't feel this way about lesbians because they don't have such a mentality that women aren't "supposed" to be with another woman. Same way with straight girls who love gay sex but hate basic same-sex affection between two women. Disgust is different than discomfort. Being uncomfortable with something means, OK, well, this is something I don't expect or is new to me, but it shows you this person is open to calming down, and realizing, OK, well, that's not a big deal.
Disgust is a major line in the sand basically saying THIS IS WRONG. So whether people realize it or not, when you call basic same-sex affection disgusting, you're calling it wrong.