The west has been in a soft-polygamy system since no-fault divorce was instigated. The only reason the illusion of classical marriage limps on is because the vestiges of the old system, mainly the ideals of classical marriage, remain attractive for those who are uninformed of the reality on the ground.
Thus, young average men enter into marriage with women assuming lifelong monogamy, and are surprised to find their wives spending time with above average men.
Females of our species have no benefit to be gained by settling for second best genetic material. They want the best, and don't care who provides the provisioning for those offspring. Cuckolding is a powerful strategy for females to have their cake and eat it too, which is why it is so widespread in nature.
No advanced civilization has ever survived the onset of polygamy [1]. Will the west be an exception?
We live in interesting times.