I think I have a solution.
Get the government out. We don't need a legal definition of marriage.
It seems like we do, because (for instance) of the overcomplicated tax code. So rather than respond to unnecessary complexity with more unnecessary complexity, get rid of it entirely.
We can still have civil laws regarding parental rights. It's not an intellectually taxing fix to come up with provisions regarding inheritance and medical visitation rights (hint: start and end with individual liberty to decide).
Leave marriage to the church. If the Catholic church doesn't want to marry two women, they can go to a more liberal denomination. Or get a nondenominational ceremony. Or whatever.
Once we've eliminated the legal ramifications, then I don't give a rat's tail what strangers do. And since it doesn't implicate law in any way, or connote governmental approval (or disapproval), I can't fathom why anyone else would, either.