Much of the resistance to gay marriage comes from religious institutions who feel the government is overstepping it's bounds. In my opinion, the simplest way to diffuse the issue is to separate marriage as a legal institution from marriage as a religious institution.
By changing the name of legal marriage, you clarify the distinction between church and state, and allay any concerns that the government is attempting to regulate religious institutions. You make clear what many already know: A marriage in a county clerk's office is different to a marriage in front of an altar.