The fact that you have it bass ackwards, wraith.
Marriage is a governmental institution first, that some people add their religion's doctrinal cerimonies to.
One can certainly get married without clergy involved, one must obtain a marriage license through their state, and if a couple chooses to have a religious cerimony one must utilize someone authorized by the state.
No religion has ever held title to the name, or practice, of marriage. If this were the case, then Hindus, Athiests, non-Abrahamics, etc. would not be able to get hitched in the predominatly Christian US.
Since marriage is a governmental institution, and a portion of the Christian community does not want to be associated with same-gendered marriages, it is up to those Christians to come up with a different term.