Yup, I think we should change the whole notion of marriage. Basically, the government would provide sets of contracts that grant certain privileges for certain responsibilities, like tax benefits for sharing financial responsibility. People can pick and choose among the various contracts, and there could be a "marriage" bundle that roughly corresponds to today's notion of marriage. Marriage than becomes a religious ceremony that people are free to define themselves, separately from any legal commitments.

This way you don't need prenups or whatever, you only sign the documents each party is comfortable with. If you're in a polyamorous relationship, you might combine finances with half of your partners, share hospital visitation rights with a separate half, etc, and you could marry all or some of them. Custody of children would be between biological parents or, if waived, assigned legal parents/guardians based on the contracts signed.

That's a bit complicated, but it would make things a lot more flexible. Individual contracts could be limited in number of people involved, but you could choose to sign different contracts with different people.