Monogamy is pushed by the state for reasons religious and social.

The religious is obvious.

The social is more complex, and yet more simple. Despite the evolutionary imperative for the species to breed from it's "best" or "fittest", a society of breeding elite surrounded by a disenfranchised rabble is not likely to remain stable. A society where most of the populace has a partner is likely to be ultimately more productive and "happy" and controllable.

It also creates a framework where pockets of nonconformance can exist with minimal impact on society in general.

Hereby endeth the speculation.

__________________
Calm, quiet, smooth, devastating