evolution is blind. It only cares about now, not the past, not the future.

In my view, the monogamy is the result of social evolution. It maximizes the productivity of overall society. The best analogy is resource allocation--allocating males and females.

It makes no sense to view evolution at individual bases. For any particular individual, it always fares better to spread genes or choose the best genes available. But the society as a whole is not the sum of individuals.