The purpose of the marriage contract is to (a) improve the lives of the participants, and to (b) improve the ability to successfully succor any dependents born to or adopted by the principals. Any combination of willing participants is equally valid; from the traditional pair, to a group of shared husbands and wives, to a basketball team or whatever. The makeup of the participants need only be limited by the imaginations of those participants as long as conditions (a) and (b) are met.

If you really want to improve the institution of marriage, however, the revised marriage contract needs to have a fixed expiration date. If the contract is not renewed after X number of years, it dissolves automatically according to pre-established terms. It would be far less traumatic and stressful, both for society and for the participants I would think, if a dying marriage was allowed to expire gracefully than to force participants to noisily murder it, or to just keep the suffering marriage on eternal life-support.