Perhaps the question should be "What is it about girls that makes guys more desirable..." =)
I've seen this happen both ways, so I don't think the phenomenon is gender-specific. Maybe the fact that someone's taken is a motivation to act before it's too late. I think this is tied to competitiveness. People start thinking "What do they have that I don't?" or "Huh. I never noticed them before, but now that you mention it..." I think a bigger reason, though, is that people often act (and feel) more confident and open once they're off the market, and that can make them more attractive. Occasionally I think that people become slightly overconfident and imagine themselves being more sought after than they actually are. (Like the girls who frantically flash their rings at every guy who says hello to them.) Those are my guesses, anyway.