You really think that....I am sure they were smart enough to link sexual activity with a new member of the tribe a few months later...
Yes, I do. Nine months is a long time to make a connection, and in people who were having sex all of the time (look at other primate species for my reasoning on this), it simply was too difficult to connect the two. The discovery that sex leads to procreation was discovered at the dawn of agriculture.
Large beasts were domesticated to provide labor on the farm, and were penned up to prevent escape. When one pens up a female animal without access to a male animal, you have a "barren" female. Surely, some farmer noticed that. It could also have been noticed that the offspring of a female animal resembled the male animal she was penned up with, and not so much the male animal she was not able to have sex with.
In order to have the concept of "my son", a man realized that he had to have exclusive sexual access to a female in order to be sure he was supporting his own. This was all known long before genetics, or microscopes to find sperm in semen, or any other scientific knowledge that we have today. Hence, all of the religions that developed in the agricultural era have needed some sort of sexual restraint on women, even though they often winked at sexual license by at least some men. Even the Old Testament mentions "wives and concubines" being had by the kings of ancient Israel.
as well they were in tune with the seasons/nature...
Possibly, in the temperate zones, but what about the tropical regions of this planet, where humanity was most often found, that didn't have true seasons? In gatherer societies, food would have been plentiful year round. In fact, explorers have encountered such people who did not link sex and procreation.