"How Do I Love Three?"

I'm a 40-something woman, living with my 50-something male partner. Our relationship is slightly open, in that every Tuesday, we each go out separately and "do whatever with whomever." I have lived up to my part of this, but I recently discovered that my partner has not. On Tuesdays, he stays home by himself. Beyond being irritated that he's effectively been lying, I feel weird being the only one doing the open relationship thing. How do I get him to live up to our agreement?

--Poly-Annoyed

There's no fun like mandated fun. What's next, holding him at gunpoint and demanding that he enjoy miniature golf? Chances are, his lying and your feeling "weird" that things aren't all even-steven in the sexual snacking domain come out of the same place -- the evolution of cooperation and the sense of fairness that fostered it. Fairness comes down to how benefits or resources get divided between people -- whether in a balanced or imbalanced way. We evolved to get all freaked out about imbalances -- even when they're in our favor -- explain population biologist Sarah Brosnan and primatologist Frans de Waal. In fact, we are driven to equalize things "to our own detriment." But, don't get too misty-eyed about human moral nobility. They point out that it's in our self-interest to take the long view -- trying to avoid being perceived as unfair, which could kill the possibility of "continued cooperation" between ourselves and a partner. Understanding the likely evolutionary psychology behind your feeling upset could help you focus on why your partner is saying (a silent) "nope!" to the sex buffet. My guess? He loves you and wants you to have what you need. And he doesn't want you to feel uncomfortable about going out and getting it -- even if the only taboo things he's doing in bed are allowing the dog on it and clipping his fingernails and letting them ricochet around the room.