This is really not polyamory or non-monogamy in the way you’re framing it. Paying for sex outside of your marriage isn’t really non-monogamy in my book - it’s just sad.
The original argument is that monogamy is in place because men cannot get sex elsewhere. That is obviously not true. The point being that men also desire faithful long-term companionship. The ones who want casual sex or multiple partners still obtain that even in our monogamy-by-law society.