Domestic partnerships could be exploited as well I guess, so it's not necessarly about 'marriage' or even 'civil unions' which would purport to have exactly the same monetary and legal benefits.
Back in the 90s, Seattle was one of the first cities to recognize domestic partnerships in an effort to give gay partners benefits. However because of the same anti-discrimination laws and considerations, they couldnt make them *just for gays.* Anyone living together (there were a few criteria) was eligible for the benefits. My boyfriend (of many years) and I lived together and he had better benefits thru the city so we signed me up for those benefits, perfectly legally.
"Marriage" is the issue...the term. So many people object to SSM that agree with civil unions. Marriage really does mean something to some people....gay or straight (not me esp.)