Alright, so the aforementioned schema, in brief, is as follows:

The fact that the legal status of marriage can be extended or denied to some relationship forms and not others means that the legal status of marriage is inherently arbitrary in its application.

As soon as you have the debate about whether to extend legal marriage status to non-heterosexual couples, you are forced to confront the fact that this status is arbitrary. Some people think you can unring that bell by arbitrarily limiting that legal status rather than arbitrarily extending it. Maybe they're right, but I don't believe it will happen.

Now you can either take the position that it is acceptable for government to enact laws that arbitrarily extend or restrict legality to certain conditions and not others, or you can believe as a matter of principle that the law should not be applied arbitrarily.

If you take the latter position, then the government can have no role in conferring legal status on the relationships of individuals. (I'm not raising the issue of the legal and financial rights of spouses here because it seems the gay marriage debate is distinct from the issue of those rights which can be conferred through other legal structures.)

Clearly, marriage is more than just a legal institution, but its social significance is buttressed by the authority of the state. If the state is forced to cede that authority, the significance of marriage in society would slowly decline.
(I have no opinion as to whether a decline in the significance of marriage would be a favorable or unfavorable outcome.)

It is unlikely that the state will simply give up its relationship ratifying authority. However, that authority will always be arbitrary and any arbitrary application of authority loses legitimacy over time.