Perhaps part of the problem is girls feeling like they have to conform to the helpless, non-sexually dominant feminine stereotype - i.e. if a girl wanted to have casual sex she might not say for fear of being called a slag?
The opposite may be true for guys, they may feel pressure to live up to the butch, sexually dominant, non-emotional stereotype.
Of course it's probably a way more subtle process than that, and people probably don't even realise they're doing it.