Who benefits from stable, monogamous relationships?
I think both society at large and the couple themselves benefit. Because of those societal benefits I'm happy to see gay couple given my implicit qualified approval in the form of civil unions. In the end though, gay couples are getting a greater benefit from their stability than society is.
Stable heterosexual couples bring greater societal benefits through child-rearing. That role deserves and justifies unqualified approval in the form of legal marriage.
"I disapprove of what they do, but approve of the way they do it."
From what fundamental principle then do you derive monogamy.