Assume for a moment that civil unions are legally equivalent to marriage, and that the only difference between the two is the name.
Keeping in mind that Civil Unions are open to hetero folks. Who if they wished, could get CU'd as an objection to the exclusion of gay folks in Marriage.
Why then argue to call it marriage?
EDIT - I really didn't want to post this, as I know I'm going to take insane amounts of shit for it, but I really want an answer to that question.