I would also like to know what “pro-marriage” means. I have yet to see a single candidate in any election ever in this country that is “anti-marriage”. So I am forced to assume that this is a reference to gay marriage, a practice which, if you endorse its legality, most certainly puts you in a position of being pro-marriage, by definition.
I would even go so far as to argue that advocating the legality of gay marriage is about as pro-marriage as you can get, far more so than any position characterized by a desire to prevent gays from marrying, once again, by definition.