By context I mean the vast changes in the overall culture regarding the issue of homosexuality. This is not irrelevant.
In order to make the argument that polygamist marriage would follow from gay marriage, then I'd say you'd need to demonstrate that polygamists had made significant cultural inroads and gained social support over the years in the same way gays have. You could apply that same standard to any other special interest group regarding marriage. If you assert that there's a risk of opening the door to incestuous marriage or marriage between a man and his goat... then I'd ask you to show me the advocacy groups and political lobbies for these minorities and examine their political weight.