From my understanding the most "natural" sexual marketplace is that women will go for the top 20% of men and the top men will likely have harems.

Why is it a bad system? Why is sexual communism and equal distribution of women preferable by today's standards? The monogamous model was created mostly because of material conditions. In the future, the robots will be doing most of the menial labor anyway and technology will make economic reforms necessary. The economic reforms and technological advancement will definitely affect the SMP too.

Isn't it better for our species if women are reproducing with the "top" men anyway? Just a thought.