The Purpose of Sex in Humans?

by Paul Sunstone

Thursday, July 09, 2009 at 10:27 PM EDT

It’s easy to believe the natural purpose of sex is reproduction. Offspring, after all, are the single most spectacular result of sex. Many of us seem bedazzled by that fact. Consequently, some of us seem to have got the notion the natural purpose of sex is reproduction and that sex without the possibility of reproduction is at best selfish indulgence and at worse perversion.

A Muslim friend tells me Islam forbids birth control on the grounds that sex is divinely ordained for reproduction. I have heard much the same thing from Catholic friends, and from fundamentalist Protestants.

Even though I think my friends are wrong (and they think I am wrong), I am not going to argue with them in this post because I am concerned here only with the notion that nature has ordained a purpose for sex. The notion some god has ordained a purpose for sex is in some crucial ways quite different from the notion nature has ordained a purpose for sex.

Maybe the best way to approach the question of whether nature has ordained a purpose for sex is to ask what human sex would look like if it’s primary purpose were indeed reproduction.

If that were the case, then I think we might expect human sex to more closely resemble sex in most other mammals. Mammalian females — including human females — are not always fertile. In fact, females are more likely to be infertile than they are to be fertile. Given that fact, if the natural purpose of sex was reproduction, then it would not make sense to have sex when the female was infertile. It would instead make sense to have sex only when the female was fertile. And in most mammals, that is the case — sex occurs almost exclusively when the female is in estrus or heat. That’s the phase during her reproductive cycle when she is most likely to be fertile.

Yet, humans are not like most other mammals. Instead of mating only when the female is in estrus or heat, humans mate both in and out of estrus. That one fact, when properly understood, by itself lays to rest the notion that reproduction is the primary natural purpose of sex in humans. For why would human evolution discard the arrangement of mating only when the female was most likely to be fertile, if the primary natural purpose of human sex was reproduction?

Even though that one fact is all that’s needed here, there are several other facts that also make nonsense of the notion the primary purpose of human sex is reproduction. Among them in this: Estrus in most mammals is highly marked. That is, females clearly signal they are fertile. But in humans, estrus is almost indistinguishable from other, non-fertile, phases of the reproductive cycle. Why would human females no longer signal when they were fertile if reproduction were the primary purpose of human sex?

Again, post-pubic human females are sexy even when they are not fertile. It seems that in most mammals, the females undergo changes that make them sexually attractive — or sexy — only during estrus. However, a case can be made that human females are more or less constantly sexy. For instance, a female chimp’s rump swells up during estrus. On the other hand, a human female’s rump swells up at puberty and remains constantly swollen thereafter, regardless of where she is in her reproductive cycle. But why would human females be more or less constantly sexy if the primary purpose of sex were reproduction?

Those are three factual reasons for rejecting the notion that nature has ordained reproduction as the primary purpose of human sex. But there is also a philosophical reason to reject the notion.

The argument that nature has ordained reproduction as the primary purpose of sex rests on the assumption that nature has purposes. Yet how can nature have purposes? In order to have a purpose, you must have a will, and nature has no will. So, instead of speaking of the purpose of sex, one might speak of the function of sex.

It seems pretty obvious that one of the functions of sex in humans is reproduction. But it is equally obvious that another function of sex in humans is bonding. And there might be other functions that are less obvious. But whatever functions there are to human sex, we should be very clear there are at least three factual reasons and one philosophical reason to refrain from stating that the primary purpose of human sex is reproduction.