Robert P. George points to evidence that polyamory is moving toward mainstream approval, though it’s far from it at the present moment. It’s significant, he says, that the LGBT movement is embracing polyamorists. Notice also, he says, the media framing of reporting on polyamory. It’s following the same script they followed on SSM. Once you give up the idea that marriage is founded on conjugal principle, you have no strong moral grounds from which to condemn polyamorist arrangements among consenting adults, says George. How do you tell them that they cannot have what they desire, if sexual and emotional desire itself is sufficient basis for expanding marriage rights.  He expands on that here:

This explains, I believe, why polyamorist groups are welcome at Pride parades, for example. They are regarded from within the movement as the next sexual minority in line for liberation and social acceptance. Indeed, they are already accepted within the movement. It is understood that the appeal they are making is not different in substance from the appeal made by those sexual minorities whose progress has come ahead of theirs. Sure, they had to be pushed into the closet for a while, in order to blunt the arguments advanced by defenders of the conjugal conception of marriage and the moral norms associated with it. But that is no longer necessary. They can be mainstreamed using the same script.

Are polyamorous relationships often unstable? Sure. But so are many same-sex relationships. So are many opposite-sex relationships. Is there any a priori reason to suppose that among people who view marriage as essentially sexual-romantic companionship or domestic partnership and who construct their lives and relationships in line with that view, polyamorous partnerships will be more unstable than monogamous ones?  Is there any evidence for such a supposition?  And even if there is, what would follow from it for the claims of those who believe their personal fulfillment is to be found in multiple partner unions, even if statistically such unions tend to be less stable than other types?

Nobody has a good answer to this, at least none that I’ve seen. They just say, “That will never happen,” or, dismissively, “Slippery slope!”, as if there were no such thing as slippery slopes.