I would not object to calling it a civil union, but we know the IRS would not treat these equally, not would insurance companies, hospitals or banks, unless the law demanded it.
Of course, since I am neither gay nor a great fan of marriage, my personal views are irrelevant.
If you want to give people the right to be married, just do it. It harms no one, it makes no demands of any church. It it might be construed that way, write into the law that it will not oblige any clergyman to wed anyone against his will.