I think it's because heterosexual relationships are the norm and you're used to seeing them, whereas you don't really see gay couples make out that much, so you get uneasy when you see something you're not used to. I'm gay myself and even I feel a bit strange seeing two guys making out or even two girls - not because I think it's wrong or disgusting but because I don't see it very often. If I saw it as often as heterosexual couples I wouldn't feel uneasy. As for you liking girl on girl, well that has been hyper-sexualised in the media and you probably watch girl on girl porn so that's no surprise.