Aside from the more interesting discussion inserted into this thread I’ll mention that I find it doubtful that anyone thinks sex and pregnancy aren’t related. Plenty of people now believe all sorts of myths about conception, but not that sex isn’t involved, and I doubt anyone going back to our early tool making ancestors could ignore the obvious connection.
I kind of think of those myths as the equivalent of “the moon is made of green cheese” stories. Everyone tells them, part of the folklore, but I’m sure everyone except the children know better.