In my experience, when I watch two people kiss (assuming it's porn, or a situation where I can get turned on from watching), I get turned on by putting myself in the place of one of them. The feeling of a woman's lips on mine (or elsewhere! !) gives me pleasure, so I can soon imagine it's me doing (or receiving) the kissing. A man's lips on mine is of no interest, thank you. (And, yes, I think there is a double standard involved where I feel, seeing men kiss - ugh! not for me, thanks.)
Here endeth my two penn'orth.