To start with, I'll reiterate that I am not qualified enough here, and you seem to know more than me. :)
Since you asked still, below is my current understanding:
1. Biological evolution itself depends on spreading of genes. So the big picture cannot be an evolutionary path where having more than one partner is less optimal, whether for reasons of STD or otherwise*. Unless it is so bad that had we not picked up on monogamy, the homo sapiens population may have phased out altogether. Most species are continuing to live by nature, do not follow monogamy and are still thriving. May be we have increased our average lifespan by mitigating STD via monogamy practices, while the lifespan of animals continues at the baseline.
* I have read and noted in other comments here that emotional love between men and women lasts for about seven years. I suspect that nature purposely evolved us this way to keep the parents together when the children are very young, and once children are sufficiently grown up, the parents open themselves up to new partners.
2. In the modern times, we are more capable of solving such a challenge than we were in the ancient times. Surely, COVID challenged us, however, we have made it work out overall. (No disrespect intended for the people who have lost loved ones, suffered on health, finances, or otherwise.) If monogamy is to be taken out, more precautions would naturally come in to compensate for a negative impact of STD. We however did not have provisions for such precautions in the ancient times.
3. Likewise, while the progress for vaccines may be slow, that would not have been if we did not have the practice of monogamy. In other words, if we were to redesign things for today, we may have worked the optimal solution to be non-monogamy plus vaccines.
I have more thoughts to write.