First, that isn't going to happen, so why waste time on fantasy.
Second, that legalizes all marriage, including polygamy, because everything and everyone is a religion if they want to be, and Islam is definitely one, but you can start a new one tomorrow.
Thomas Jefferson, who dealt with marriage law as an attorney, lamented the religious takeover of marriage when the all powerful single church made it a religious ceremony instead of a civic ceremony, during the 5th century, when Rome was starting to lose control to the Popes.
America does not have a state religion, so to say that every church and every religion should define marriage is of course impossible, since Americans can marry without belonging to any religion, besides, that would simply mean that there is no definition at all because there is no single religion or church.
Thomas Jefferson can be wrong.