I think you miss his point. I think he meant that perhaps the reason why few of these relationships resemble monogamous marriage is because they are not allowed to be married. You can't expect them to look a certain way if they cannot be that way, no?
So ... you think that public sanction is that powerful ?
So powerful that it will change gay relationships so that they much more closely resemble heterosexual marriage ?
Those same-sex couples who have been allowed to marry in certain European countries(Norway and sweden) have already shown a divorce rate 1.57 to 2.56 times higher than heterosexual couples.
This data was compiled from 1995-2002.
http://www.imapp.org
I can guarantee you with a high degree of certitude that the same will happen here as well. The divorce rate would balloon and the institution of marriage would be further weakened.