Whoever said that marriage is a sacrament had it right, too. I quite agree, and the term marriage ought to be exclusive to religious ceremonies. Let the churches and synagogues and mosques and temples and such determine what constitutes marriage among their members, in whatever gender or quantity considered moral by the standards of that religion. Leave the government out of it.
Drop marriage from anything to do with government. In it's place, have household unions or somesuch, defined by legal contracts equivalent to those understood by the current marriage ones. Make them as difficult to dissolve as a divorce is, and while we're at it, why not permit them to be for limited terms if the parties desire? (Yeah, I love Heinlien.)
Poly groups? Sure. It's not for me but it suits some people. Put them all under a household contract.
Let family members form household groups. If two sisters live together with assets in common, treat them like any other pair of people in any other household group.
There are problems, of course. Taxes - but I think the tax system needs serious overhaul anyway. Healthcare benefits for family members - but what, really is the difference between covering a three-adult household and a two adult one child household? Sorting it all out would be a neat trick, but in the end, worthwhile, I think.
And by the way, it isn't about gay rights with me. Quite frankly, the term makes me cringe, since I don't particularly feel that people have rights based on gender, orientation, race, or religion. It's about recognizing that a society consisting of family household units of various varieties is inherently stronger than a society with a this-or-nothing approach.