I realize that the conversation has moved on from this by now, but I wanted to add something because it relates so perfectly to something I was reading just last night.
Correct, if I thought sex was purely for procreation. But I don't. I think it's primary purpose is procreation.
This quote is thoroughly wrong but illustrates nicely what I just learned in the book The Third Chimpanzee by Jared Diamond, author of Collapse and Guns, Germs, and Steel (you may have heard of him; MacArthur Fellow, awarded National Medal of Science, professor, etc.).
From the chapter titled "The Evolution of Human Sexuality"
Whatever the main biological function of human copulation, it isn't conception, which is just an occasional by-product. ... In no species besides humans has the purpose of copulation become so unrelated to conception, or the rhythm method so unsuited for contraception."
Most mammals are sexually inactive most of the time. They copulate only when the female is in estrus - i.e., when she is ovulating and capable of being fertilized. Female mammals apparently "know" when they are ovulating, for they solicit copulation then by presenting their genitals towards males. Lest a male miss the point, many female primates go further: the area around the vagina, plus in some species the buttocks and breasts, swells up an turns red, pink, or blue....
Human sexual cycles are quite different. The human female maintains her sexual receptivity more or less constantly, instead of having it sharply confined to a short estrus phase. Indeed, despite numerous studies aimed at settling whether a woman's receptivity varies at all through her cycle, there is still no agreement about the answer - nor about the cycle phase when receptivity is maximal if it does vary.
Our concealed ovulation, constant receptivity, and brief fertile period in each menstrual cycle ensure that most copulation by humans are at the wrong time for conception.
Summary: if the main purpose of human copulation were indeed reproduction, we would only mate when the female is fertile and it would be damn obvious to both females and males when that fertile window is. As someone who has purposefully studied my cycle in order to become pregnant, I can tell you that I can only now, after 5 years, be certain when I have ovulated, and that is often with the aid of tools such as ovulation predictor kits. Furthermore, from a purely biological point of view, copulation comes at a price, both in time and energy spent having sex rather than looking for food, making yourself distracted and vulnerable to predators, and for the male, making and ejecting sperm also takes energy. In most other animals whose sole purpose is procreation, all of this energy expenditure is carefully regulated to get the most bang for the buck, so to speak. We humans are clear outliers in this trend because we do not mate primarily for reproductive purposes.