“Contraception is wrong because it’s a deliberate violation of the design God built into the human race, often referred to as ‘natural law.’ The natural law purpose of sex is procreation. …sexual pleasure within marriage becomes unnatural, and even harmful to the spouses, when it is used in a way that deliberately excludes the basic purpose of sex, which is procreation.” —http://www.catholic.com/library/Birth_Control.asp

Among Christian religions, it is a common teaching that the “natural” purpose of sex is for the creation of children, and that any other reason for sex is sinful and a subversion of God’s law. This belief underlies a great deal of Christian attitudes toward sex, including the religious right’s condemnation of homosexuality and extramarital sex and the Roman Catholic church’s opposition to birth control.

However, while the overtly theocratic manifestations of this belief – denying civil rights to gay couples, denying emergency contraception to rape victims, attempting to ban pornography, refusing to teach young adults accurate information about sex, opposing the development of vaccines for STDs – have been attacked, and rightfully so, as far as I am aware the belief itself has rarely been challenged. That is unfortunate, because as in many other things, this is an area where the religious right stands on very shaky ground. This post will accordingly examine the facts, or lack thereof, supporting this belief.

First of all, it would seem that if the purpose of sex – the reason for sex’s existence – is procreation, then all or most acts of sex should result in pregnancy. However, this is not the case among human beings. Consider what anthropologist Jared Diamond has to say in his book The Third Chimpanzee:

…even young newlyweds who omit contraception and make love at maximum frequency have only a 28 percent probability of conception per menstrual cycle. Animal breeders would be in despair if a prize cow had such low fertility, but in fact they can schedule a single artificial insemination so that the cow has a 75 percent chance of being fertilized! (p.77)

As Diamond points out, human beings are far less fertile than most animal species. And, please note, the numbers Diamond quotes are for young newlyweds. Human fertility declines steeply as we age, especially in women but also in men. Female fertility peaks between the ages of 20 and 24, begins to decline as early as the late 20s, and drops off more sharply during the 30s. By age 45, the vast majority of women are infertile (1, 2). Even if an older woman does become pregnant, her chance of miscarriage remains substantially higher than a younger woman’s.

The Bible itself says that the human lifespan is between seventy and eighty years (Psalms 90:10), and given the advances in lifespan brought about by improved nutrition and modern medicine, we can safely use the higher estimate. We should then ask why, if sex is intended for procreation, it is all but impossible to use it for that purpose for about half of our natural lives. Even during the biologically brief window of maximum fertility, our rate of conception is significantly lower than that of most other animals. If sex’s only or primary purpose is procreation, then it would seem to be a badly designed mechanism indeed, considering how inefficient it is for that purpose.

There is yet another biological argument that suggests that the primary purpose of sex is not procreation. In our species, females do not experience estrus – that is to say, human women do not go into heat. Virtually every other species of mammal does, and in many of those species, the onset of estrus is marked by conspicuous physical changes that advertise the female’s sexual availability. For example, female chimpanzees‘ genitals become swollen and bright pink when they are ovulating, a sign that is obvious at a glance.

By contrast, ovulation in humans is not just not advertised, it is concealed: there are no external physical or behavioral signs that reliably indicate when a woman is capable of becoming pregnant. In addition, the length of the female menstrual cycle exhibits much more variation than cycles of estrus in other mammals, making accurate prediction difficult. (Ironically, although the rhythm method is unreliable for humans, it would work great for gorillas.) These facts, as Jared Diamond points out, ensure that most human sex acts will take place at the wrong time for fertilization. Again: if the religious right is correct and sex was designed by God primarily for procreation, why would God make it so difficult to use it for that purpose? Why would he design human beings so that we must have sex many times to stand a good chance of initiating a pregnancy?

These difficulties persist as long as one clings to the view that the only reason for sex is procreation. But if we discard that assumption, the matter comes into clear focus. An alternative explanation that accounts for the facts much better is that sex has two primary purposes: for procreation and also for pair bonding. And while pair bonding strengthens the family structure needed to raise healthy children, that is not its only purpose. In nature, it can also be used as a stress reliever, to strengthen group cohesion, as social currency, and simply for pleasure. Even homosexual sex exists in nature. Are animals violating “natural law” when they use sex for these purposes?

The concealed ovulation of females, and the constant receptivity of both genders, not just allows but encourages human beings to have sex more often than is strictly necessary for procreation. One might say that we are designed this way. If we abide by the religious right’s simplistic arguments about what is natural or what we are meant to do, we are not just guided but compelled to the conclusions drawn in this essay.

However, we, unlike other animals, are not rigidly bound by the dictates of evolution and instinct. We are not required to abide by what is “natural”. (If the religious right were consistent, they would also oppose eyeglasses and surgery for appendicitis – and priestly celibacy! – since those things are just as “unnatural” as condoms or birth control pills.) The purpose of sex is whatever we decide it is, and so long as we do not use it in ways that harm others or infringe on their equal right of self-determination, that decision is our right to freely make.