IMO human nature will be the final determinant. We know that same sex relationships are as old as humanity. Despite some rare occasions, previous large scale political structures that were stable were built on heteronormative foundations. However, as the possibilites for stable political structures grew to include the possibilities for more social heterogeneity, a constant minority has become better able to realize its natural expression. Whether or not a polyamorous minority suppressed by current structures exists remains to be seen. I haven't encountered much evidence of it. That said, there's no doubt that many on the hetero/homo sexual spectrum assumed roles that were more hetero than their inclination due to pressure. If anti-polyamorous pressure is lifted, there may be a realization of some sort. But, even if that comes to pass, it's pretty clear that it will be some time in coming.