I'll reframe this for you: in a liberal society, we default to open social policies rather than closed ones. There's no moral or principled legal basis for the

I'll reframe this for you: in a liberal society, we default to open social policies rather than closed ones. There's no moral or principled legal basis for the civil right of marriage being extended to only straight couples. Restricting polygamy does have a principled legal basis: it's historically associated with subjugation, and you don't have a civil right to subjugate others. If and when that association vanishes (i.e., society is convinced that all parties are able to competently consent), then there will be no legal basis for excluding them from the civil right of marriage. Your "ick" response is not a valid basis.