A woman can only bear a single man's child in a 9 month period. A woman with a large number of male suiters is theorectially less appealing, since the chance of successful mating (where successful -> results in offspring in the near future) is very low (assuming each man has an equal chance, it's essentially 1/(#ofSuiters) ). In reality, the energy cost of impregnating a woman is so low that it doesn't really matter that much, since he doesn't actually have to bear the child.

A man can impregnate many woman in this same period of time. All of these woman have a good chance of creating offspring, so the main concern of an individual woman is finding a man who will be able to provide for her child. This is a complex requirement - it isn't necessarily sufficient to just look at a man to gauge his ability to secure resources. Also, the energy cost of becoming pregnant is extremely high for a woman. Therefore, the woman reacts based on a large number of variables. She also assumes that other woman are searching for a man based on similar requirements, so the fact that a man is surrounded by other woman increases his value as a potential mate.

I realize that this simplifies things quite a bit, but I think the fundamental argument is sound.