Sex is complicated. The why, the who, the how, rarely so simple as the love myths would like you to believe.
For instance a study was just released in New Scientist suggesting that primates may treat sex as a commodity, trading a good knit-pick for poon. In fact, after grooming a female the likelihood of her "engaging in sexual activity" is three times more likely than were the grooming to not have occured.
"And as with other commodities, the value of sex is affected by supply and demand factors: A male would spend more time grooming a female if there were fewer females in the vicinity."
Now, I'm not saying this study isn't highly insightful in its analysis of biological relationships. But really, do we need all the capitalist terminology? Down with the dollar!!!