There is no substitute for marriage, either one is married or he is not. If the state contrives some contractual entity with the exact-same provisions as marriage, then it is marriage, whatever the state might call it.
But the state may not play a semantic shell game, and attempt to appease same-sex couples with a cheap imitation of marriage law, a separate but equal approach is as illegal as the denial of access to marriage law itself.
Words have meanings and those meanings have deep reverberations for those who hold them dear or revile them.
A rose by any other name may still be a rose. Would you demand that Christians be forced to refer to themselves only as infidels? Use the N Word in the wrong company and you may find a lesson in sensitivity forthcoming.
If the word applied to this is offensive to some, then use a different word. Why must it be necessary to offend some to remedy others?