Which, unlike same-sex marriage, is an institution with deep roots both in America (the Mormons were forced to give up this sacrament as a condition of statehood) and in the majority of world cultures, where it ranges from condoned to celebrated.
Without getting unduly personal, let's say that I have a stake in that question being resolved. I know several triples living quietly among us; they face the same kind of problems (child custody, hospital visitation, inheritance rights) as same-sex couples faced prior to this decision.
What the polygamists of the nation lack is a powerful lobby. One may hope that nonetheless, reason and freedom will prevail here as well.
EDIT: nation, not world. Worldwide the situation is different. America is suffering from its Christian legacy here. Most Christian countries are adamant about denying this right to their citizens.