>https://www.tandfonline.com/doi/abs/10.1080/07418825.2016.1216153
>HLM regression models indicate that men who transition to a monogamous, or less competitive, mode of sexual behavior (fewer partners since last wave), reduce their risk for violence. The same results were not replicated for females.
That version of non-monogamy is pretty different from the standard polyamorous deal. It seems to be more about churn than about multiple simultaneous partners. The properties don't obviously transfer.
>jealousy
From what I've heard, that's a major thing that keeps some people away from polyamory. People who engage in it already have lower than typical sexual jealousy and are conscious of it.
>STI rates
This is orders of magnitude less of a problem than in the past, assuming everyone gets tested regularly. If this was an important reason not to do polyamory in the past then that makes polyamory correspondingly less problematic today.

Different societal conditions mean that different tradeoffs become viable. If theory predicts that something is irrational but people do it anyway, always consider that maybe it's the theory that's wrong.

And don't forget that there's no reason for evolution to align with morality, and that people are not fitness maximizers but adaptation executors. The most "evo-aware" thing to do wouldn't be to maximize sexual success but to become a sperm donor and stop caring.