I've never understood this double standard:
Straight guys seem to like it when "lesbians" are doing things because "twice the women twice the HOT."
This interest ends when said guys learn that said "lesbians" DON'T want to have sex with males, though.
On the other hand, seeing two guys do anything remotely homosexual with each other... they tend to take it personally for some strange reason.
For the record: I too used to think "Two girls = hot, Two guys = ewww" back when I was a teenager. This stopped after I overheard some of the girls in my class talking how they didn't mind homosexual men (I think one of them specifically said that she kinda understood them since guys are hot.) but felt that lesbians were a "threat."
That got me thinking about the subject as it was a mirror version of what I felt and I think that was the very point when I realised that there is nothing wrong with being gay and furthermore I shouldn't be squicked by their existence.
And I think that part of straight men finding lesbians hot is that they might be uncomfortable watching another man have sex, perhaps feeling their sexuality threatened if there is a guy involved in the act or something.