When it comes to marriage, Martin Luther agrees with the CommunistsMarriage is a civic matter. It is really not, together with all its circumstances, the business of the church. It is so only when a matter of conscience is involved. " (What Luther Says CPH 1959, Vol. II, page 885)
Unless you're Catholic, there's plenty of "Christian" theology for Protestants to cover their fuzzy little buns with if they want to go along with Civil Unions, Queer Marriages, or whatever else the State wants to call the legal arrangements they oversee.
That's a major part of the problem you run into when you argue against such State sponsored decadence. There are an awful lot of people have already had it drummed into their pointy little heads that Christians have no business saying anything about what is and isn't a legal marriage. They've been taught that Christians should love eveyone and let them all be equal up to and including marrying whomever they want. Far more often than not, it was their "upstanding" Protestant pastors who were doing the drumming in league with their favorite fellow faggot.