Just added it to my previous post but figured I'd mention it in a bit more detail. The studies I mentioned - detailed in Why Men Don't Listen and Women Can't Read Maps (a fantastic and highly insightful read, I fully reccomend it) - show that women are likely to find some kind of attraction to both genders, making them commonly bisexual at least to a minor degree, which would explain their willingness to kiss, although perhaps not go further. The same studies showed that guys were far less likely to be bisexual - but more likely to be homosexual. Essentially, girls are more likely to be happy with either gender, while guys are more likely to either be sexually interested guys only or girls only. This would explain why you often see girls making out with other girls, but are less likely to see straight males making out with other guys. It also explains why you may have noticed seeing more homosexual males than homosexual females.

Seriously, it's well worth reading that book. Great read.

EDIT: GB, man, I'm getting married. I don't need to worry about that crap anymore.