marriage grants legal rights, privileges, benefits, obligations and duties to both married-partners. There's approximately 1,100 federal laws that kick in if one is married. There's approximately 300 or so State laws (depending upon which State) that kick in to marrieds-only.
Now, that is what it's all about. Only marriage grants these rights, not so-called "civil unions." That's the impediment to your theory.
If there were so-called "civil unions" state-to-state, there is no guarantee that State A would grant the same or similar legal rights than would State B, etc. In other words, there'd be no uniformity in "civil unions" as there is in law to marriage; and there would be no way to format a uniformity in law as to all the States.
As for federal law, so-called "civil unions" is a zero. A huge zero. Can you imagine the federal congress re-writing over 1,100 laws to apply equally to "civil unions" as those laws apply to marriage? Of course not.
In application it would be like this:
1.) I get married. My marriage is legally enforceable and recognizable in law in
all the States including the territories and the federal government and across the entire planet. My family (in marriage)
including my children may rely upon and obtain privileges, duties, obligations, and burdens of marriage across my State, and all States and territories in America, plus foreign nation recognition. (However, it's still questionable as to what weight a same-sex marriage may have in some foreign nations.)
2.) However, if I have a "civil union," I don't know what my State grants me and it varies State-to-State and it's completely unenforceable State-to-State, and it's a zero both federally and internationally. My children will suffer from not having the protections of state, federal, and foreign laws of married parents. My grandchildren would also suffer as do my children.
If you are so inclined to read further on this issue, try this:
http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=221&topic_id=2401&mesg_id=2604 . . . . . . . . . . . . . . . . . .