http://forums.sjgames.com/showpost.php?p=1145468&postcount=80
That isn't borne out in reality. Modern settings, with their age-of-consent laws and mass-media moralism, encourage some of the highest levels of sex avoidance seen in human history; assertions to the contrary are nearly always the propaganda of said mass-media moralists. The majority of myth and history is dedicated to recounting who shagged whom and ended up knocked up. And it isn't as if intrinsically non-procreative sex acts were invented recently . . . there are prehistoric carvings depicting most of them. Unless you use a Clintonesque definition of "having sex," I think it's a given that people past puberty had sex about as often in era X as in era Y, for arbitrary X and Y.
Here's another thought on that: My father is a genealogist – one who has in fact been decorated by my country for his work in that area, not an amateur. As he tells it, part of why it's hard to trace lineage back past a certain point in most societies is because of records: people weren't literate, documents weren't durable, and/or record-keeping was reserved for the upper classes. But an equally big part of it is because truth be told, outside of a few times and places that were ultra-conservative about marriage, most people were bastards. To first order, humans have sex casually and "marry" as a temporary arrangement when that leads to pregnancy. It isn't a popular view of the species with modern religious folk and those who pretend to blue blood, but genealogy and genetics support it rather unequivocally.