They see gay couples as more egalitarian. With rare exceptions, straight women assume gay couples are always sweet to each other, clean, and loving. Contrast that with what they see in their own parents to get varying degrees of intensity.
>>8505111
What is it about lesbians that straight men find so hot?
It's the same kind of question – they find gay couples cute because, well, if a guy is hot to them, then two guys kissing and holding hands must be super cute.