Debate spawned by same-sex marriage is deeply rooted in religion. As we have a separation of church and state, it stands to reason that this word should be returned to religious institutions.
Any two legal adults can currently form any of the following government-recognized bodies: Corporation, Trust, LLC, etc. These provide modifications of liability, tax incentives, rights of successorship, and more.
A marriage, in today's terms, is merely another iteration of this theme. Yet, it has been used throughout history to exchange property (women) or to bar unions based on race.
It is time for a federally recognized civil union to be instituted requiring no more regulation than the aforementioned. Simply allow any two legal adults to enter into a civil union. Leave the word "marriage" out.