Matthew has a stance on marriage—gay or straight—guaranteed to annoy anyone who’s never read Locke (and probably some of them, too). I won’t spoil it; just go RTWT™.
James Joyner likes the idea in principle, but is concerned about some of the implementation of the details. Given the burgenoning industry surrounding pre-nuptial agreements, I’m not sure we’re that far from solving those problems. And, in terms of inheritance, avoiding probate is largely a matter of having a proper will, and with the inheritance tax on the way out the door the potential tax issues are greatly simplified for people in most states.
Overall, it seems to this non-lawyer like most of the practical benefits of marriage (excepting the tax benefits) are largely duplicated in existing contract law; the trick is to create a “civil union” contract that contains those provisions—durable power of attorney, the method of disposal of assets upon dissolution, etc.