Heterosexual marriage benefits society by producing the nezt generation. The stability provided by a STABLE family develops good and productive members of society. It's not the relationship or the love between two people that Society cares about, but the results.
Is it really?
Lots of homosexual couples last far longer (and are more stable) than heterosexual marriages. Homosexuals do raise children - through adoption or artificial insemination. My own three-parent bisexual family raised two stable, fully-functioning members of society - myself and my sister. Homosexuals are as much a part of his society as heterosexuals, and society has as much to gain by providing them support.
I would very much like to see a study that shows children raised by a comitted homosexual couple are any less stable than those raised by comitted heterosexual parents. And it would be greatly beneficial to the pre-existing homosexual family units today to obtain the recognition - and Bennies - of marriage.
Personally? I don't care what you call it. So long as it is completely equal in the eyes of the law - it cannot be excluded from something provided to marriage. So long as it is recognized across state lines, gets the same tax benefits and/or hits, has visa benefits, inheritance rights, etc. If it is 100% identical in every way, I don't care what it's called.
Unfortunately, all domestic partnerships I am aware of fall short of this. At the very least, they do not provide visa arrangements for unions where one individual is a foreign national. And including every single marriage benefit in a contractual agreement is similarialy problematic - you're going to leave out something, and especially when it comes to visitation and inheritance, they can be trumped by situations that wouldn't faze a marriage.
I know you said that when you asked couples why they wanted to get married, it was all about the bennies, not about love or raising a family or comittment. But no one ever questions why a hetero couple wants to get married, not with the same inherent doubt of legitimacy. Of COURSE couples who want to get married wish to solidify their comittment in the eyes of the law and/or their God. Of course they want to be a family.
But these things are implicit in simply saying, We want to be married. So they skip over that and go for something more concrete, more accessible. Feelings and dreams are too easily brushed aside. Besides, technically, you CAN love one another and commit and raise a family without getting married. It's just more difficult, and vulnerable, and painful. Benefits are something you can point to and have it stand up as a solid obtainable goal of marriage recognition.