I believe his point is that consensual oppression shouldn't be allowed. We don't allow people to voluntarily become slaves, for example.
On the contrary, I know of several people who have been allowed to voluntarily become slaves. What we don't do is legally enforce contracts that involve chattel slavery or homicide, but "consensual oppression" is a pretty broad brush and one that I have a front-row seat to on a daily basis.
Firstly, that's what polygamy means (as opposed to polyandry).
No, that's not what it means. You're confusing polygamy with polygyny.
However more importantly, excluding the endless combinations and recombinations that occur in the polyamory community in the west, polyandry only really occurs in some tribal and other traditional contexts. Polygamy, however, is very compatible with the patriarchy that constitutes most complex societies and is both far more common and, I'd argue, far more prone to abuse and exploitation and other power imbalances.
So what if you allow both polyandrous and polygynous marriages?