Those are two cultures - even if they are the oldest ones. Do you absolutely know for sure how they lived 100,000 years ago is exactly the same as they live now? It's unlikely that there wouldn't have been some changes. They provide glimpses into our prehistory but there are not perfect snapshots of the past, in part because even cultures that are very similar and live in the same region may have different mores and social elements.

If you are likely only going to live to 35 (the typical lifespan for a Paleolithic person), it makes more sense to have children earlier so that you can raise them before you die.

And how can you have sexual monogamy and at the same time engage in sex without many constraints? Did you mean that they have social monogamy, meaning something that approximates marriage, and then don't worry too much about sexual constraints? That would make a lot more sense.

Patriarchy perverted the natural course of sexuality. It is only when that arose 6-9 thousand years ago that all of a sudden people cared about paternity. If everyone is taking care of everyone else in the group, it hardly matters who the father of child is. Those religions and the patriarchal values they embody, only came later on, but I do agree that Western colonialism spread that and imposed their values far and wide.