sleeping with people not your husband is a risk-seeking behavior that also increases genetic success for women. But the way you've phrased it is so loaded in "the way things are" that it's hard to see just how prevalent the cultural norms are ingrained.

Sleeping with more than one man may decrease a woman's time-spent-not-pregnant by a small factor. But no matter how many women a man is already sleeping with, adding one more will always increase his reproductive success by the same amount. The potential gain due to "cheating" is much higher for a man's genes than for a woman's because he has no downtime.

That has nothing to do with cultural norms, it's just biology.

There are certainly examples in the animal kingdom -- female dominant, where females maximize their genetic 'success' by successfully having children of many males.

Yes, the inverted world scenario could have happened with humans, as it has in a few cases (lemurs, bonobos, and hyenas).

But I think you're misinterpreting why it comes about in those cases. There is no suggestion that it emerges because it in any way increases the probability of a woman getting pregnant; to the contrary, that's the easy part, most researchers think female dominance is more related to ensuring that adequate resources are available to ensure the survival of the mother and offspring, which is a different issue altogether.

In those types of societies, we should (and do!) find that male reproductive risk seeking is more muted, because such risk seeking no longer carries with it significant evolutionary benefit - with females in control of the hierarchy and (limited) resources, and quickly impregnated when they're able, a male maximizes his potential success by staying alive, playing nice, and working to make sure the children survive. There's no contradiction there with what I've suggested: if you change the environmental constraints (social, developmental, and physical), the optimal evolutionary responses to those constraints change, too.

Such a society would be quite different in many ways, and the evolved differences between the sexes would be much smaller (lemurs display very minor sex differences compared to most mammals). But we should not pretend that the very real evolutionary pressures humans evolved in response to are not there just because things worked out the opposite way for a tiny fraction of mammals, and in the absence of a female dominant social structure, male risk seeking tends to have a significant evolutionary payoff. When we see evidence of it in humans, I don't see anything wrong with recognizing the incentives that lead to its expression.

So a better question to ask is "why did that not happen with homo sapiens sapiens?"

To put it bluntly, we can ask the question, but it's trivial to answer: female dominance is such an aberration in the mammalian world that it's probably selected against. 3 known examples out of more than 5000 species is under .1%, small enough that we can assume that in the evolutionary space and ecological habitats that we exist, female dominance has not been an optimal reproductive strategy. Even if it was neutral, we should be seeing more instances due to drift.

That's not a moral value judgment, or a justification, or anything like that - we're not slaves to our genes, we have the ability to overcome our instincts and emotions, and we should do so whenever they lead us astray. I think most reasonable people these days agree that dominance in any form is a primitive trait whose expression we should be trying to eradicate from our society.

But the current trend in some circles is to claim that in humans, most sex differences are purely social constructs, and hence are not influenced in any meaningful way by our genes or evolution. Which is bollocks - genetics and society are highly intertwined in most species, performing a delicate coevolutionary dance where one can't move without the other compensating accordingly, and this is something that any evolutionary biologist sees every day. Like nature vs. nurture, socialization vs. genetics is an argument for the fools, they happen together, and it's supremely naive to argue for one to the exclusion of the other.