You can still be monogamous if some women have more children, not every woman needs to have 2 children to achieve that average. A woman can easily have 4 children and suddenly half of the women can also be childless and still achieve a 2 children average.

The whole idea that child bearing should be perfectly and equally distributed across the whole population is a construct of the social welfare state, enabled by subsidies, and is just an artificial form of equality like any other. As such I think it should be examined and debated like everything else, and not taken for granted as self-evident or "natural".