The issue, as I see it, comes down to the difference in perception of "marriage" vs. "civil union" or domestic partnership".
Marriage is a ceremony that is meant to be carried out in a church, and is then recognized by the government as a legal contract.
Civil union / domestic partnership should (key word there) carry the same weight, without the church affiliation.
Do they carry the same legal weight? If that answer is "yes" then is arument should have been dead on arrival.
As one of those "religi-type" peoples, I disagree with the idea of SSM, but fully support the ideas of civil unions and such. I think it opens up a pandoras box of all kinds of problems when the government attempts to force the church to change its standards of marriage. I know there all kinds of problems already (the whole "sanctimony" of marriage argument has lost a lot of its luster) but there should still be this separation between the two.
If a church wants to allow it, then they should be allowed to, but they should not be forced to change their standards, because the government says so.