This might be a good question to post to one of the science subreddits. One of the ways scientists gauge this question is testicle size with testicle size indicating how much competition the sperm have to face. Humans are between gorillas (which are very monogamous) and chimps (which are almost free for all in mating) when it comes to testicle size. This suggests that while in our evolutionary history we haven't been totally monogamous we also haven't been having a lot of partners. I'm sure one of the science subreddits would have those who could go into a lot more detail than I can. Hope this gives you somewhere to start.

The concept panzerkampfwagen mentioned is described in Matt Ridley's book The Red Queen: Sex and the Evolution of Human Nature. Perhaps you should locate a copy. I would also recommend hitting up a science sub for more details. In my evolutionary biology class last we discussed the idea and I recall a chart showing that humans are literally right on the regression line that relates promiscuity to testicle size, so that we can sort of go either way, being generally monogamous, but with adultery being a regular occurrence.