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".

> 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 median is close to the average in kids per women in the developed world. Having four kids is harsh on many women’s bodies; eg my grandma had 8 kids, but she died in her late 50s. While no doubt some couples will want to go for four kids or more, I don’t think it’s ethical to encourage it over a more standard 2 something for most couples.