I think that's because an acknowledgement that the kind of marriage one selects has an "environmental impact" immediately brings the sacred cow of gay marriage

I think that's because an acknowledgement that the kind of marriage one selects has an "environmental impact" immediately brings the sacred cow of gay marriage into question. When it really shouldn't. You still have a fundamental right to select the type of relationship that's right for you, but you do have a responsibility to be mindful of its effect on society. This doesn't just mean ban stuff. Perhaps for polygyny, that would translate into making the relationship non-exclusive, rather than choosing against it altogether.