Different explanation, and some of this ain't pretty. (Apologies if this is male-centric, but it's probably quite similar for women.) Richer people (or, at least, socioeconomically comfortable people) establish masculinity and effectiveness by building successful families. If you're happy at work and your kids are doing great and you're intellectually engaged, one woman is enough for most men. That obscene sex drive…
Human sexuality is very complex, and has had evolutionary pressure on it from many different directions... but that's not how the science works. I'll let Slate Star Codex explain (http://slatestarcodex.com/2014/08/20/ozys-anti-heartiste-faq...):
"Is it true that some humans are r-selected, which means they’re promiscuous and want to have lots of children, and some humans are k-selected, which means they invest in a few children?
No.
For one thing, r/k selection theory was shown to be a vast oversimplification of life-history evolution forty years ago. For another thing, it applies on a species level, not on an individual level. Humans as a species are absurdly k-selected. How many species can you think of that have young that are totally helpless for their first decade of life? r/k selection theory would argue that we wouldn’t have casual sex, because humans are not remotely r-selected on any conceivable level! Clearly, this is not the case, which ought to make one wonder how applicable this is to human beings anyway."
Here is a paper going into the details of the science: http://www2.hawaii.edu/~taylor/z652/Reznicketal.pdf