what do you think an unbiased heterosexual man should feel stumbling upon two men kissing in public?
Generally when I stumble upon a couple kissing in public, if I think anything at all about it, it's something along the lines of "Oh, good for them, they're in love," and I'm happy for them.
I'm not sure why the genders of the participants would be relevant. I've got my own gender preferences for relationships (I'm a straight man) but again, I'm not involved, so my preferences for myself aren't relevant to the situation.
In a larger sense, one of the dumbest things you can do is form opinions for no reason. You aren't obligated to form an opinion on everything you come across.
It's literally self-destructive to feel some sort of negative feeling about this. You don't have to. Why would you want to?
Nothing? Maybe an aww? What do you feel when you see animals nuzzling each other? Or kids in the park doing their thing?
If you felt disgust or fear in response to seeing a man and a woman of a specific race kiss, how would you respond to yourself?