Plain and simple.
If civil unions could give the same benefits to gay and lesbian couples that heterosexual married couples enjoy (even to someone like Britney Spears, who doesn't seem to grasp that marriage is not some damn thing to do on a drunken weekend in Vegas), well great, fine. I'd be 100% in favor of civil unions.
OR...if the government were to get out of marriage entirely...if heterosexual couples were to enter into civil unions as well as gay and lesbian couples...reserving marriage as a stricly religious right, without conferring any legal benefits whatsoever, regardless of sexual orientation...then, hell, I could get behind civil unions. But we all know that neither of those scenarios are ever going to happen.
We gay and lesbian people deserve the same rights that you do. There are no legal reasons to deny us those rights. As the Supreme Courts of Masaachusetts and Canada have so ruled.