And the reason why they want the civil act of marriage rather than a civil act of union would be...?<quoted text>
It is the civil, not religious, act of marriage that most homosexuals want.
You are not considered married by any state or the federal government until you receive a marriage license. A religious ceremony is not required by any state to have a valid marriage.
Ever heard a minister say "by the powers invested in me by the state of ______ I now pronounce you...."? The person officiating over the ceremony is acting as a agent of the state.
Assuming all legal rights and responsibilities were exactly the same in a civil union and a civil marriage, why fight for the right to be married versus the right to be partners?