I'm not saying that people shouldn't be monogamous if they want to be, I'm just pointing out the biological underpinnings of this discrepancy in male and female orgasms.
As for what happens in menopause, I'm sure that varies, but age 50 (6 years ago) was when my husband and I opened up our marriage (at my suggestion). I'm at a stage in my life where I don't have to take care of others quite so much and can focus more on what I want, and on my pleasure. My sexual appetite has only increased with age, and although my husband is still my number 1 and one of the best lovers I've ever had, I'm very interested in other lovers and love them also, not so different from some indigenous women it seems.
The Aché of Paraguay consider two people "married" if they share a hut together, although people do shift huts with little fanfare. However, couples who have children tend to stay together while they are raising them. Women who are beyond child-bearing years have an average of 13 "husbands."