The Family Research Council (FRC) filed a supporting brief. How nice to have others joining the fray. The FRC does not really have anything new to add, but it has been said that right makes might.
Opponents of traditional marriage values appear to have their priorities reversed. They seek to challenge existing laws, and should bear the burden of proof that what they advocate offers something superior to established tradition. This is what judicial review seems to refuse to recognize. Proponents of "equality" confuse it with "equivalence." They appear to be looking for a reason to hold a prolonged free-for-all party, and now propose to redefine "marriage" in those terms.
But the judiciary has already upheld the argument that government has a compelling interest in supporting traditional marriage. Together with the Supreme Court ruling that recognizes the compelling interest of states, there is really not much doubt that the traditional marriage argument stands on firm ground.
The FRC, along with the state of Utah, has no obligation to further demonstrate the virtue of traditional marriage. They are not trying to invent something new. Nor is the defense of marriage obligated to demonstrate that alternatives are inferior or wrong. Rather, it is up to the challengers of tradition to show why their innovative and trendy new toy represents a superior way to promote the general welfare.