the laws of economy taught us that a market works well when there's a certain balance between demand and supply. when such balance doesn't exist, a market tends to fail. I believe this might be the case of our free SMP that started roughly in the 60s.
Women's hypergamy (and on a lesser extent male koreogamy) create a strong imbalance between the sex demand from men and the correlated supply from women.
Besides that, the growing number of third world immigrants in the West, most notably those of muslim persuasion, will likely exercise a strong pressure on society to revert to a rigid monogamic culture to ensure a minimum sexual life to any guys since their early adulthood like it was in traditional societies. or am I exaggerating with my predictions?