For one, guys are definitely more biologically primed for sex. Testosterone (sex hormone in much higher levels in males) has been largely held responsible. Here's a recent review: http://www.ncbi.nlm....pubmed/21334362
In fact, some even believe testosterone has a direct link with aggresssive behaviours, and its not a big jump to suggest that the mre aggressive people are the ones who will be more, well...outspoken about their biological urges: http://www.ncbi.nlm....pubmed/21075538
As to the ladies, well, data does also suggest that the female brain is less geared towards "aggressive" sexual behaviour, and more selective mating. Baron-Cohen suggests females are more geared towards picking up subtle social cues than males, meaning they can more effectively assess the suitability of a mate than males can from mere social interaction.
What does this all mean? It means there definitely IS a biological explanation for increased promiscuity in males, but whether or not this has direct relevance on why society welcomes it requires extra consideration. Certainly, just because girls are predisposed to be more selective about their mating partners doesn't really explain why society don't saction such behaviour either, especially when a proportion of society is made up of horny males who might think they'd "benefit" if things went the other way round!
I'm pleased to see everyone is adamant on the social explanation though. I think its about a 70-30 nurture-nature split here. The role of inborn biology definitely is not as insignificant as everyone seems to think, but its definitely not set in stone either. Indeed, the abovementioned male characterisitcs changes were found to be inducible in females in certain environments, for example in the context of stress!