Thanks, I appreciate it and I'll look for it although I already see a lot of potential problems with this theory.
Many animals only mate in season, even though sex undoubtedly feels good any time of the year. Why would that be so if there wasn't a biological drive to propagate?
Chimps and bonobos (and some others of our primate cousins) have sex for the fun of it, often with others of their own gender, but females don't mate with their adult sons. Why would that be, if there weren't some instinct that speaks to what creates a better genetic result?
Why would gorillas and other animals that fight for a harem of females need to do that? If sex is primarily for enjoyment or social bonding, why not go the bonobo route and just let everyone copulate with whomever they want?
Why does the coronal ridge of the human penis serve to scoop out the semen that might already be present in the vagina, if not to try to mitigate genetic competitors?
Why do so many other animals have penises that serve to do a similar thing? Lots of birds have what are essentially bottle-brush penises to remove the genetic material left by competitors.
Certainly, there are other reasons for having sex, particularly in primates, than just continuing the genetic line, but there's actually quite a bit of scientific evidence, some of which I've outlined here, that says that animals (of which humans are one species) have a biological drive to propagate their species.
I'll agree with you that in many instances, Darwin was pretty off base about how that takes place, but that's kind of a tertiary topic.