Okay, so how about this:
we make a distinction between legal marriage (a legal contract, recognized by the state, courts, IRS, lawyers, and all legal actions) and a religious ritual, which only has meaning, status, and bearing on the specific religion that performs/administers/ordains it. Then religions can define their ritual in any way they choose — only for the purposes of reproduction and populating the earth with their own brand of offspring. Between 1 man and x-number of women (depending on the religion.) With provisions forbidding birth control (see the reproduction rider, above.) With specifications like obedience. Submission. Division of labor. Division of child-rearing duties. Division of funds. Property ownership.
And the scope and reach of each religious ritual is limited to that particular church and that specific couple.
It would mean couples would have to get a legal license that could be filled out by anyone recognized by the state (like we have now) and filed with the courts — and the specific ritual could be tied to (or not) one of the religions.
That actually sounds like a separation of religions and the government. Unlike the soupy sticky mess we have now, where religion seems to think it can impose its rituals and rules on legal documents and institutions.