But.. but.. heterosexuality as we know it in humans is NOT good design if its purpose is reproduction.

Now, if you want to see good reproductive design, look at dogs. They have a peaceful life until the female is ovulating and then she comes into heat, stimulating the male to defy all obstacles and fertilize her. She does this only when she's at a capable reproductive age and every heat would naturally tend to be a pregnancy. Every single sex act is directed at making puppies and has no other baggage.

Now look at humans. We are in continuous heat from the age of 11 or 12 to near death, even when we are long past reproduction. People go into heat long before they are in a good position to be reproductive. Couples are naturally inclined to have sex far more than once per pregnancy. A substantial minority of people are naturally differentiated to have sexual desire only in non-reproductive combinations. Males produce so much semen that if they can't have sex they must either masturbate regularly or begin to soil their sleeping places.

It's perfectly obvious, then, that human sex is purposed as much for intense communication and bonding as for reproduction. It goes FAR beyond reproductive need. In any case, to maintain the "sex is for reproduction" argument and then allow couples with one non-reproductive partner, like a wife who's had a hysterectomy, to continue having sex is just zany. It can't remotely be justified. What is the "purpose" of this sex? I have always joked that it must be sufficient for heterosexual couples to imitate the purpose of sex if they are not reproducing at the time (I know the pope would disagree, and say you must have sex only in order to reproduce, but in this he has no true followers that I know of.) Well, so reproduction is good, and reproduction charades are good... but loving communication is not good. What's wrong with this picture? Meanwhile, what do humans have conjunct with their sexuality that dogs don't have? Love! Enduring, supportive love! And what does God enjoin to us? Love! Enduring, supportive love!

(Biologically, infant dogs are highly vulnerable for about 5 months. Infant humans are highly vulnerable for at least 10 years. Clearly, long before our ancestors had monotheistic religion, something was needed to make human couples stick together and raise these young a long time. Ongoing, constant sexual interest, not for reproduction per se but for bonding, must therefore be an evolved biological response. Since non-reproductives, whether found in same-sex or opposite-sex pairs, can make a strong contribution in a social species, there is no biological barrier to these people having a mutually supporting bond relationship just as with the reproductives. Indeed, humans constantly out-reproduce their food resources and begin either to starve or to steal resources from neighbours, and the presence of some non-reproductive, contributory members of societies is clearly potentially beneficial.)

And it's not as if there is any lack of biblical quotations supporting love for one's partner independent of the pure reproductive urge. Song of Solomon, for example.

This is one bit of ancient Greek teleological speculation that really needs to be uprooted. Either we demand that infertile straight couples desist from sex or we broaden our notion of sexual purpose.