I found this gem on The Dawn Patrol:
Precisely because sex does not always create children, yet it always holds the promise of creating children, that sex stands for something greater than itself. Because sex is designed to produce children, yet does not always produce them, the act is transformed from a simple biological action into… there was no other word for it… poetry.That is why sex is MORE than just biology.
Because sex contains not a hard reality, but only a future promise, it becomes a promise, the promise of the man to the woman "I will be with you always, even if this does produce that for which it is designed."
And by this act, the man gives himself not just to the woman, he gives himself primarily to the not-yet-conceived child.