this shouldn't come as a shock since we are both currently convinced we are in love with people who we rationally know to be wildly inappropriate

During sex, women secrete a hormone called oxytocin. This is the same hormone that we experience when we are nursing our babies. Some experts refer to oxytocin as the attachment hormone, because this hormone causes us to both relax and connect with the person we are with. In the aftermath of sex, we relax and commit to our sex partners. While we are nursing, we relax and connect with our babies.

In both these situations, oxytocin, the bonding hormone, has survival value. Connecting with our babies helps us to take pleasure in being with them and caring for them, even at times when it wouldn't ordinarily be very pleasant, (like in the middle of the night.) Connecting with our sex partners increases the chances that we will stay together long enough to raise the baby to adulthood.

For men, the pathways to bonding are a little different. We have all heard, ad naseum, actually, the argument that men and women have distinct mating strategies. Men seek to impregnate as many women as possible, and invest as little as possible in each child, while women seek to nurture each child to full maturity. We are less familiar with the obvious point that men must compete for women.

Women prefer men who will be faithful providers for them and their children. Women's strategy compels men to be more faithful, less philandering than they would otherwise like to be. The man's strategy places him at war with himself. The evolutionary payoff for men to settle down with particular women is the assurance that the children he invests in are indeed his own. At least one psychologist argues that jealousy helps men to connect with their sexual partners. Possessiveness helps the man settle down.

A man doesn't feel jealous or possessive toward every woman he sees, or even toward every woman he finds attractive. He feels jealous over women he has had sex with. This feeling of jealousy is in some ways a nuisance for the woman. She might feel that the man is trying to control her activity, or even her thoughts and feelings. And make no mistake about it: men do sometimes go over the line and become obsessively jealous, even dangerously jealous. But, one thing is for sure. A woman knows that she matters to a guy who gets jealous.

Possessiveness is the dark side of male attachment. The bright side of the very same tendency to attach is loyalty. Men are capable of heroic loyalty, to their wives and children, to their teams, companies and countries. Our culture indulges in so much male-bashing that we sometimes overlook this salutary fact about the male half of our species.

All this makes perfect biological sense. This is Mother Nature's way of making sure we stick together long enough to have fun, make a baby, and then work together long enough to raise the baby to adulthood.

This tendency to attach to each other means that in a very real sense, causal sex isn't even possible. There is a reason guys come unhinged when they find out their "friend with benefits" has another friend. There is a reason girls sit by the phone, wondering whether the guy they hooked up with the previous night will ever call them again. The basic desire to connect with our sex partners is built into our physiology.