I don't understand the concept of "civil unions". Supposedly they're meant as a compromise relationship that would allow gays to get the benefits of marriage without using the same name, right?
Well what's to prevent me from civilly unioning with my roommate to get free health coverage from his work and to save money on taxes? Or, for that matter, what's to keep me from unioning with a family member or a business partner for similar financial reasons? Once the financial transaction in question is completed, we could simply dissolve the union, thereby freeing ourselves to form other unions as it became advantageous.
Would civil unions convey legal spousal privilege? If so, then criminals could simply union to avoid testifying against each other. Likewise, such privilege could be used by parents unioning with their children to cover sexual abuse. The list of potential problems seems endless to me, and I don't see any clear criteria that could be used to draw a line.
It would certainly be absurd to require two people to somehow prove that they're gay before allowing them to enter a "civil union". Most states allow for minors to get married with their parents' permission, and so I see no reason to think that children would not be allowed to enter into civil unions, possibly even without parental consent. If a girl can get an abortion without parental notification, then why can't she get civilly unioned? Similarly, parents are not allowed to marry their children, but does a civil union necessarily require or expect there to be sexual activity between the two partners? If not, then there's no reason not to allow parents to union with their kids.
The complications go on and on, and any inclusions or exclusions will end up being entirely arbitrary. The well-defined structure of marriage has been the building block of civilization for all of known history; creating an institution of "civil union" would necessarily undermine that order. Proponents may or may not admit it, but I think that undermining the current social fabric is one of their main intentions.