You make a lot of assumptions here. Sometimes the answer is simple and people are not sexually compatible, were never good at having sex together and rushed into a long term relationship. I've seen this more in people who were further along the 'sexual repression' spectrum (i.e. following religious advice about relationships and marriage).

Sometimes I think cheating and divorce is as much the norm as mating for life. I was reading a study recently that found that sexual intimacy steadily decreases over time in a long term relationship, and only seemed to go back up again if divorce/new partner happened. I will dig it out if anyone is interested.

There is a taboo around non-monogamy which I think is detrimental to many people, and certainly a lot of people make multiple partners work, despite all the barriers to doing this in most societies. It isn't a sticking plaster for other issues, that I agree though. If anything it needs the participants to understand well what they want and to be excellent communicators - something we can probably all work on.