Legally speaking. I don’t think polygamy/group marriages would be that hard of a sell to the courts based upon the elimination of a requirement for a marriage to be between people of two different genders. With the redefinition of marriage, all that really remains is that it is between 2 people. There are sufficient religions and world cultures where polygamy is standard that a person (with sufficient financial backing … because it’d be a tough fight) could probably challenge the “two people” requirement. I’m thinking you could come at from an “equal protection under the law” angle and a “freedom of religion” angle.