“In your world, Islam and gays and atheists, and anyone and everyone defines legal marriage.”
That sounds like your world to me. Gays, atheists, Muslims, and anyone and everyone can vote to determine how the state recognizes marriage right now, that’s how the state comes up with its definition. To the state, marriage is simply whatever judges, pols, or the voting public decide it is at any one time. Sometimes it coincides with the actual definition, sometimes they come up with impossibilities like ‘gay marriage.’
Does the state define marriage for you? I doubt if it does. I bet your faith would go with its own definition if the state tried to punish it for not accepting ‘gay marriage’ or some other impossibility the state is calling marriage at the time.
Freegards
In other words there is no such thing as marriage, it is without form or definition, it is whatever anyone says it is.
That should be interesting when it comes to law, and military couples and interstate travel.