Yes, blinder, I very much stand by my statement that there is no obvious connection between sex and babies.
You have sex. Nothing happens immediately. You have sex some more, maybe with the same man, maybe someone else. You have lots of sex, and nothing happens.
Then your period stops; and I'll agree that our ancestors would have been aware of menstruation occurring more or less regularly, and what the cessation of menses meant.
Then, many, many weeks after you had sex, and quite possibly while you're still having sex regularly, your belly begins to swell, the child grows, and several moons later you give birth.
You're probably still having sex regularly throughout pregnancy.
So how is it obvious that one particular act of sex caused the baby?