Let me state the obvious first and get it out of the way. Straight men like girls. They find girls attractive. So if instead of one girl, you put two in front of us - what happens? You've doubled the number of boobs, the attractiveness, the fun. It's also an obvious choice between either one girl, touching herself and two girls...touching each other!
Mostly, we don't even think about it in any more depth than this. But when we do, there are a few other reasons we're a bit, uh, obsessed with girl on girl sex. It starts with it kind of being a bit of a taboo in our society. Okay, a BIG taboo. But even then, many of our sexual kinks do originate from the same place - they're all unconventional (and some even considered "unnatural”) things.
And this kind of leads into the next one - if a girl is actually involved in a sexual situation with another girl, we automatically assume that she's a bit on the wilder side of things. Because, see, she's obviously willing to experiment in bed and go beyond other people's definition of pleasure-giving methods. And that's really, really hot. Like, seriously, smokin' hot.
Of course, despite having mentioned all of the above, we have to accept that there is a certain level of jealousy involved as well. Why is that girl with another girl? Why isn't she with me? Why isn't she doing all of that with me? Sure, we get that this might make us sound a bit childish, but we can't help it. Especially if we have feelings for one of the girls involved in all of the action going on. Because, duh, we want some too - preferably with someone we like.
Aaaaaand this leads us to the final thing - our never-ending hope that maybe, just maybe, we can be part of the scenario and turn the girl on girl scenario into a threesome fairytale. At some point in his life, EVERY guy dreams of a threesome...and he dreams of it with two girls. He's in all likelihood lying if he denies this. So, yeah, part of the excitement of watching two girls together is imagining our own selves as the third character in the scene!