I have recently been reminded of a particular hypothesis that I have: previous to human consciousness, assuming some sort of evolution, our ancestors had a predictable and relatively tame sexuality. Most definitely the women were monogamous. You could argue either way for men- in fact, perhaps it would be best to reserve judgement, since the environment and population would do the telling as to whether the men had a mate, several mates, or no mates at all. Indeed, from a historical perspective, most men don't have a chance.
Anyway, before consciousness became what it is in humans, that is before it became substantive enough for the being to be considered human, the being's environment and hormones would act in concert to enforce normative behavior.
With consciousness, a new possibility emerged: humans could now reflect, recollect, imagine, and start to change their hormone profiles to a degree not known before. Suddenly the normative behavior, while it still tended to be baseline, could be changed. Especially in the beginning, I suspect this would be a combination of opportunity in the present and the remembrance of something particularly thrilling in the past- the feeling of the past get overlaid onto the present, imagination kicks in, and suddenly the conscious focus on this imagination leads to changes in the hormone profile. Suddenly, things that never were considered could be considered, and done.
So, many things were tried, and many people died. Humanity built up a series of morals around sexuality meant to ensure that there was a next generation and that the next generation would be healthy.
There has been research in this area I regard as laughable, impractical, and likely just a continuation of the progressive narrative. Women are only fertile for so long, children take a lot of time to mature, and comparisons made between us and other primates suggest strongly that modern erotica written back into the past doesn't make any sense. They didn't have the pill, nor did they have the state to stand in as father. Even among tribes and small bands, it would become very obvious incest would cause serious problems, and, in general, too much deviancy leads to illness and death. They didn't have antibiotics or modern health care either.
Yes, sure, folks could have gotten up to some odd stuff, but chances are, those folks are not our ancestors, but an evolutionary dead end.