JGC:

How does same sex marriage argues in favor of consanguinous marriage to any greater extent than traditional marriage already does, with the sole proviso being that it argues the cousin you elect to marry should be of the opposite gender?

The reason marriage is considered a pairing and not any other combination of people is because there are two sexes and marriage is the union of two people of the opposite sex. If you are going to eliminate the basis of marriage as being the pairing of two people of the opposite sex, then you destroy the logical basis for marriage being a pairing at all, as opposed to any other combination. If the only basis of marriage is consent, then any number of people who agree to get married to each other would equally apply.

You could arbitrarily (for the time being) continue to restrict marriage to only between two people, but on what basis? We don't restrict the number of people who can be parties to any other legal contract.