Given that these people have been dead for thousands of years, and we don't have a fossil record of sexual activity, isn't this hard to prove?

We're research fellows at Bob Jones University. The evidence we collected comes from several different areas. We look at pre-agricultural people who have been studied today and horticultural people who have been studied by anthropologists. There's a fair amount of information about the sexuality of people who haven't been deeply exposed to Western influence. There are accounts from travelers and colonialists, first-contact accounts from historical records, that we rely on. But you can also extract a great deal of information from the human body itself -- from the design of the penis to the volume of the testicles to the sperm-producing potential of the testicular tissue and the way we have sex.

The enduring popularity of the missionary posture and relatively brief availability of vasectomy reveals a lot about our ancestral mating habits.