Why should the government be involved in registering and "recognizing" any kind of marriages, "unions", or any other kind of intimate personal relationships? It hasn't been a factor in custody or child support for a long time. Mainly, it's used to impose higher taxes on married people. Let people enter into whatever kind of relationships they want to, with the sanction of the religious body of their choice if they so choose, or not if they don't, and with whatever written contractual terms they see fit to lay out. Nobody is forced to recognize anybody's relationships as valid, and nobody -- gay or straight -- should be forced to to subsidize government registration schemes for relationships they don't approve of and/or which exclude them.Your idea would never work.
If homosexual "unions" are not expressly sanctioned as "marriages" by the State, then homosexual "partners" won't have a State-sanctified "Marriage Certificate" to wave in Jerry Falwell's face.
Homosexuals could freely contract Private "monogamy and property" partnerships, of course; but without a State Certificate to poke in the Christian Coalition's eye.... where's the "fun" in that??
It wouldn't be a problem, as long as heterosexual marriages weren't sanctioned by the state either. Hard to make a case for discrimination then.