remove governmental favoritism toward married people, that solves the problems of people trying to game the system with fake marriages
otherwise, a marriage contract is just a contract, and we have the precedent of prenuptials to indicate that one size does not fit all. i'd like my state to honor a wide variety of marriage contracts. from persian short term marriages to heinlein line marriages. each religion can have their lawyers draw up standard forms for their particular beliefs on the nature of marriage, as can secular groups, and rich folk will custom write theirs just like they do with their prenups now.
as long as people willingly sign up for the marriage on the terms they agree to, whoever or whatever they are, the only involvement of the state should be to allow the courts to enforce the contract