OK, the psychological take on same-sex kissing...
 

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! :D !) 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. :)

Women are generally more affectionate with each other in public anyway. I think that, and the fact that mainstream porn tends to feature more f/f action is why people are more uncomfortable at the sight of two men kissing.

I'll agree with Gus Aspar! Irish

Simple - I'm not attracted to men, I am attracted to women. Two men kissing is just odd as I don't see it very often. Two women kissing is beautiful (a) because I'm a straight male and (b) because women (and everyone, male or female, knows this) are far more beautiful than men.