From Scientific American:
Women are much choosier than men when it comes to romance. This is well known, but the reason for this gender difference is unclear. Evolutionary psychologists think it is because back in prehistoric times “dating” was much riskier for women. Men who made an ill-advised choice in the ancient version of a singles bar simply had one lousy night. Women who chose unwisely could end up facing years of motherhood without the critical help that a stable partner would have provided.
That is less true today, yet women remain much more selective. Is this difference a vestige of our early ancestry? Or might it be totally unrelated to reproductive risk, the result of something more modern and mundane? A couple of Northwestern University psychologists, Eli J. Finkel and Paul W. Eastwick, decided to explore this question in an unusual laboratory: a real-life speed-dating event.