^I'm with ILN. The benefits of legal marriage should never have been assigned to them by the government. The government shouldn't have its hands in our relationships. You can do whatever the hell you want. If I don't like it, that's great, you do you man. But we don't need the government to tell us this. The only reasons people care are either economic, which as I said shouldn't be in it (and furthermore I believe are accessible outside of marriage. This process should be made easier, ie joint custody of children, assets, etc. as well as heirs/successors, right of attorney, whatever you call making medical decisions, etc.), and from a sense of equality, which should be taken care of if the government abolishes marriage. If you want to give that to six people, go ahead. I would limit this to consenting humans and adults.
That way, if you want to get married in a church, go ahead! If you want to get married by a priest, good for you! If you want to get married by your grandpa in a shed, go ahead. If not, get your benefits and I don't really care.