I'm not ready for a relationship now, so I'm having a friends-with-benefits thing with this guy. He typically takes me out to eat before we hook up. However, a couple of times, he had someplace to be right afterward, so he didn't take me out to eat first. It really bothered me, and I'm not sure why. I know it's just sex; we're not dating. But I felt super-disrespected and almost cried later in the evening. I guess I felt used, which is weird because we're really "using" each other.

--Puzzled

To a guy, "just sex" is enough. You don't have to tell him he's pretty and take him to Yogurtland.

Although intellectually, "just sex" is enough for you, too, the problem is your emotions. They might just seem like a sort of wallpaper to add oomph to your mental den, but evolutionary psychologists Leda Cosmides and John Tooby explain that emotions are actually evolved motivational programs. They guide our behavior in the present according to what solved problems that recurred in our ancestral environment. Many of the threats and opportunities they help us manage are universal to male and female humans, thanks to, say, how a hungry bear isn't all that picky about which sex its double humanburger comes in.

However, in the let's-get-it-on-osphere, there's only one sex that gets pregnant and stuck with a kid to feed. So women, but not men, evolved to look for signs of a sex partner's ability and willingness to "invest." Even today, when that investment isn't there, female emotions are all "Ahem, missy!" -- making you feel bad: hurt, disrespected, used. Wanting to feel better is what motivates you to take corrective action. As anthropologist John Marshall Townsend observed about female subjects from his research: "Even when women voluntarily engaged in casual sex and expressed extremely permissive attitudes, their emotions urged them to test and evaluate investment, detect shirking and false advertising, and remedy deficiencies in investment."

And no, you can't just plead your case to your emotions with "But I'm using birth control!" Your emotions are running on very old software (predating even those early '90s AOL floppies), so as far as they're concerned, there's no such thing as sex without possible mommyhood. In other words, if you're going to make casual sex work for you, you need to see that it works for your emotions. Basically, your body is your temple, and prospective worshippers need to sacrifice a goat to the goddess -- or, at the very least, buy the lady a hamburger.