I do not recall the court decision being based on the pragmatic advantages of monogamy.
Not to mention the fact that same-sex marriage only gets majority support due increasing numbers of people not taking the concept of marriage seriously at all - strip away those who believe it is okay to have children out of wedlock (not simply errors in judgment - but on purpose) with someone you are not married to and do not even intend to marry, live together with someone you are not married to and do not even intend to ever marry, etc. - it would have very little support.