Just saw this and thought that you guys might find it interesting:
https://class.coursera.org/behavioralec ... ecture/111

I think that you'll need to sign in, but it's a very informative lecture that considers sex from an economic perspective. The highlights:
- Women have sex and men have to earn it by investing (emotionally/financially etc)
- Women generally have sex after the number of dates that they want, not the number that the guy wants
- Skirt lengths get shorter, on average, when women are competing more for men and longer when they're not (ie. more women than men or visa versa)
- Women find sexualised ads distasteful unless it implies men investing to have got the sex

Further research; google "sexual economics". Interesting field of psycology (especially here)!