This is America, people don’t have to join a Mosque or FLDS, or a gay church, or a Lutheran church to marry if they don’t want to, they don’t even have to believe in God to marry.
And they really don’t have to stay in that religion forever, and depend on it to decide on their divorce and property rights and child custody.
Then it isn’t marriage in the true sense if they are not married in the eyes of God.
And they can determine property rights, child custody already without the state sanction of marriage and divorce would become a non-issue.