[Tichy]

Easy: women can always get laid if the want to (not necessarily with whom they want to, but still). It's cheap for a man to try to pass on his genes by sleeping with a woman once. So why not give it a shot, even if the woman is not that attractive? On the other hand, for a woman it is very costly to pass on her genes.

[klipt]

That explains why men are more actively looking for casual partners, but there should still be women looking for long term partners (in fact there should be even more than men).

If there are fewer women than men on dating sites, that just shows that online dating is biased towards the casual.