How do you propose to do this? Community property and children to be specific?
A contract is a contract. Treating marriage like a corporation instead of a state sanctioned status simplifies things.
So, if two men want to buy a home together under the same conditions that a man and woman do, let them sign and bear the burden. Inheritance law, same thing. If Gary and Bruce want to have their property and interests treated as a similar contract between Joe and Sue in the event one of them dies, then so be it.
The thread of government over-involvement runs through all these things, twisting private enterprise to have rules, exceptions or biases they doesn't actually want, which complicates the marriage issue exponentially.
The only difference between a traditional marriage and a business is exactly where I'd draw the line, and without negotiation. I don't think gay couples should adopt children. That's not to say that most of gays aren't nice people who are financially stable. I think that homosexuals are generally good people with bad wiring. While I'm happy to live and let live with them as individuals in a free society, collectively, they need to realize that it's harmful to children to raise them in an environment that it takes an adult perspective to understand properly. That's a biological and evolutionary fact that is rather easy to demonstrate on the face of it, without needed to resort to a religious argument.
(All that said, I'm talking about an ideal state, there. I also realize that we're so far down the statist road that the only way to stop state enforced gay marriage, along with it's free speech restrictions and gay adoption of children, is a DOMA type constitutional amendment. While I feel that's not the right thing to do in the grand scheme of things, it's also probably the lesser of the two evils before us.)