This seems like an awful lot of effort to go to just to resolve issues with alternative relationships. Since we're discussing "marriage as it is defined in American society and legal system" in the status quo, it would seem more efficient to simply modify the wheel instead of reinventing it.
If the current definition of marriage discriminates against same-sex and non-traditional relationships, then work to change the definition. It'd almost certainly be easier than trying to convince Americans to give up the legal benefits and conveniences of government-recognized marriage.

Also, the opening statements are phrased in such a way as to make it seem like this is more an attempt to separate religion and marriage, yet the remainder seems more like an attempt to eliminate government recognition of marriage. If the former is the true intent, then there are certainly better ways to go about it. Perhaps if all couples (groups, whatever) were required to participate in a ceremony performed by a government official (Justice of The Peace, etc) in order to obtain government recognition, after which they could be married in the eyes of Deity X if they so choose. Then, we would have a system wherein marriage is completely devoid of religious meaning. Just an idea.

Updated after m_turner's reply:
Okay, at this point I'm forced to concede that changing the definition of marriage may be nigh-unworkable as a solution. But I still maintain that it'd be cakewalk compared to the task of passing a bill to revoke the legal recognition and benefits of marriage. This is especially true when you consider the fact that the same people who are blocking attempts to redefine marriage will likely block attempts to separate marriage and government simply to benefit a portion of the population that they consider to be deviants at best. And they'd probably have very strong public support as well.