1. Pleasure. It's fun. Receation that re-creates. It renews us.
2. Commitment. It's a non-verbal way of saying, "I belong permanently and exclusively to you." It unifies. It makes promises: emotional, legal, financial promises. In the movie Vanilla Sky, the charater Cameron Diaz says to Tom Cruise, "When you have sex with someone, your body makes promises to them even if you don't."
3. Procreation. Barring infertility, under ordinary circumstances, sex will bring children into the world.
The Catholic/Christian view is that sexuality is fully expressed when all three components are present in a relationship. In other words, if one of the three is intentionally missing, at the refusal of either partner, there is broken-ness. For example, in the Renassiance, marriages of convenience, for political reasons, to produce an heir, where there is no love, often turned a woman into object. (#1 and #3 are missing). If a couple has casual, recreational sex without commitment and children, both parties suffer (#2 and #3) are missing. If someone purchases sexual favors from another, then only party gets #1, the other one is missing all 3. If either or both members of a married couple refuse to have children the marriage and their sexuality suffers. (#3 is missing).
The idea here is that an intentional breakdown in the full expression of sexuality (which encompasses all 3 aspects) will de-feminize/de-masculinize the parties involved, but the effects will be stonger on the party that is intentionally removing any aspect of the full expression of sexuality.
That's the theory. Is there evidence for it?
Look at our culture: a whole generation of people confused about their gender roles.
I've seen countless couples hurt in their marriages by putting off having children for 5-10 years. And the stange part is watching both of them unable to figure out why the woman was getting harder and more calloused and the man was becoming bitter and confused.
On a biological level, believe or not, having no children in a marriage triggers buried instincts that the coupling had failed to produce offspring and therefore, pulls the couple apart. Their basic maculinity and feminity has produced no "fruit" so to speak. This is an insight from a friend of mine who is heavily into anthropology and evolutionary biology.
