There are nuances with the whole subject. Being against same-sex marriage could be the same as those who are "against it" but finesse the subject by saying it's a "states rights" issue.
That is, don't expect help with a federal definition of marriage. Don't even expect Congress to take that subject off the table of federal judges.
There are also nuances with those who hold to the "civil union but no gay-marriage" position.
The bottom line for me is that the nation has every reason to grant special consideration to heterosexual couples because they are the pro-creative units of our culture and they are the best locus of child-rearing for the society. To grant that type of UNION a special name (marriage) is logical, and it is a result of a particular function they fulfill for the culture.
Any "civil union" position that grants those same concessions to non-procreative "couples" is a waste of my tax dollars....there is absolutely no societal benefit in gay bumping and grinding.
I know that GW's position in the matter is on the side of civil unions, but at this point the subject is probably moot since Barney Frank has stated that with a Democrat congress they will pass some accommodation for same sex marriage.