This is one reason why SSM is not, objectively, marriage. As a rule*, marriage is a culturally and societally managed institution which, through natural processes, results in the next generation being produced. The protection and perpetuation of the next generation is something which society (whether tribal or governmental) has historically been deeply interested in. Rest assured that there is no natural way in which an SSM couple can contribute to producing the next generation. While virtually any relationship between two or more humans can be recognized and, therefore, named "marriage", "marriage plus surrogate", "grouping", "shacking-up", "brotherhood", "sisterhood", or whatever, only a relationship minimally limited to a male - female arrangement can in and of itself produce the next generation (I know, that last part is pretty obvious). As such, marriage between a man and a woman should be recognized as an institution which, as a rule and by design provides for the perpetuation of society; and not as, for example, an arrangement which provides one with governmental benefits.
* By "as a rule" I'm referring to an anomaly free, unencumbered, and properly functioning system. (thanks to Stand to Reason for pretty much everything I just said)