You are correct that there is no defensible rationale to prohibit polygamy if gay marriage can't be my prohibited. I've heard gay marriage advocates make various arguments, but they all boil down to the same kind of value judgements that gay marriage opponents make.
However, the Left won't push for polygamy anytime soon for one reason. Even if conservatives oppose polygamy, the biggest advocates for polygamy are culturally and politically conservative and probably the least feminist human beings in the Western world. It's Mormons in rural areas of Utah and surrounding states who think the mainstream church in Salt Lake City got soft and abandoned the theology. They aren't going to ally with God haters, and the Left is unsympathetic to them.
Last edited: Dec 16, 2022