I think polyamory won't reach mainstream acceptance at the level of gay/trans because people perceive polyamory as more a direct threat. You can believe your spouse isn't actually gay/trans way more easily than believing they don't actually wanna bang someone else.
You can live perfectly fine in the suburbs with your hubby and kids while all the gays go off and bang each other somewhere else, but if you imagine a world where your closest married neighbors occasionally date new people, you might freak out and put your hubby down on lock.