Marriage as it is defined now shouldn't be a legal matter anyway, all the legal matters like custody, property ownership, households for tax purposes, etc. shouldn't be tied to such a narrow understanding of family structure, so by that measure sure I suppose. Currently the support structure needed for such polygamist unions to be common does not exist, and given the way they often devolve into abusive situations, I don't think it's wise to support a move like that at the current time. But I suppose you have to get started from somewhere and without the push for legalization/normalization of the practice those support structures may not be built, making it a bit of an chicken and egg situation