The issue is the redefinition of the traditional term "marriage":
Main Entry: mar�·riage
Pronunciation: 'mer-ij, 'ma-rij
Function: noun
Etymology: Middle English mariage, from Anglo-French, from marier to marry
1 a (1) : the state of being united to a person of the opposite sex as husband or wife in a consensual and contractual relationship recognized by law.
Once this term is redefined, it will open the door for all other types of marriage other than non-tradional(polygamist, etc.).
The argument from the gay activists side is that if you use the term "civil unions", that we are going back to the 1950's with the use of "separate but equal."
I ,personally, am kinda neutral on the issue(but leaning towards civil unions). Having worked many years with foster kids, I found some of the best foster parents were gay, but unable to adopt because of that. I child will always do better(IMHO)with ANY type of stable parents rather than remaining in the foster care system. It's a double edged sword. Just my .02
