An argument about SSM and "open" relationships

Michael asked if someone could lay out the argument underlying Robby's posting of the New York Times article setting forth findings about a perceived tendency of same-sex couples to adopt "open" relationships, including open marriages.  I would never purport to speak for Robby, but here's my best stab at the argument:

One's view of same-sex marriage turns, at least in part, on one's estimation of how malleable the institution of marriage is.  Many people who support SSM strongly favor marriage as a foundational social good in that it promotes, among other things, mutual caregiving, stable relationships for child-rearing, and the total and exclusive self-giving of one person to another.  Don Browning, for example, argues that marriage is particularly important for males because “paternal investment in children, paternal certainty, and monogamy tend to go together.”  If SSM is understood to maintain these functions of traditional marriage, and if the gender of the participants is incidental to those functions, then maybe there's no problem.  (I'm putting to the side the ontological arguments about marriage made by Robby and others.)

But what if the gender of the participants is not incidental to those functions?  This possibility could take two forms: 1) biological -- i.e., the suggestion that men are, on average, more inclined toward sexual promiscuity, and marriage's "restraining" function may be less powerful when the marriage is made up of two men; or 2) sociological -- i.e., same-sex couples will tend to be less committed to the traditional understanding and functions of marriage.  If, say, half of same-sex couples will end up adopting an "open" marriage, does the pedagogical dimension of marital practices mean that open marriages will be more likely to gain mainstream acceptance, with monogamy viewed as simply another possible marital characteristic to be bargained for by the spouses, rather than an intrinsic part of marriage?  To wildly overstate the factual premises of the argument, if a crystal ball could show that the widespread adoption of SSM would lead, in 40 years, to nearly half of all marriages being intentionally non-monogamous, should that give SSM supporters pause?  I believe it should. 

The problem, of course, is that there is no way to know what marriage will look like in 40 years.  I'm not even sure what the state of committed same-sex relationships is right now.  The Times article reported on gay couples in San Francisco.  I'd be interested in the practices of same-sex couples in Chicago, Minneapolis, or Atlanta.  I'd also be interested in exploring cause and effect.  Does the apparent tendency of same-sex couples, at least in the Bay Area, to reject  monogamy (relative to heterosexual couples) reflect a deliberate devaluing of marriage, or does it reflect a subculture that has been excluded from the social mechanisms that help foster and encourage monogamy?  Will the monogamy gap eventually shrink if same-sex couples are included in marriage?  And would the closing of the gap be driven by rising rates of monogamy among same-sex couples or by falling rates among heterosexual couples? 

So can the practices depicted in the Times article carry the weight that SSM opponents might assign it?  I'm doubtful, though to be fair, I don't think their entire argument would rest on that depiction.  Does SSM represent changes to marriage beyond the gender of the participants, and if it does, are the changes likely to impede the essential social functions of marriage?  In my estimation, that's the crux of the argument that needs to be engaged by both opponents and proponents of SSM.