In order for their obligations to mean anything someone has to be empowered to act as an arbiter. In our nation that is the family law courts. So who would you replace them with? Would there be a method of appealing this arbitration and who would act as the ultimate arbiter? Whoever acts as the arbiter is arguably acting as a government, no matter what label you wish to apply to them.
Now it might be fun to kick around those ideas between libertarians or in science fiction books. But holding out for them and nothing else is a good way to sit on the fence, never be taken seriously and I have witnessed it being used to support the status quo of inequality. I have no idea if any of that applies to you as I am not familiar with your views.
So, I guess, my question then, is what do you suggest in the interim? Frankly, I am in favor of the courts striking down all bans against same sex marriage state or federal. DOMA is a no brainer. Even Bob Barr, the author of it, is against it now. The issue is NOT about federal benefits and I do not believe that is true for "liberals" who support this position.