I think it can be traced back to the no fault divorce. SSM Proponents point out (rightly) that marriage isn't taken seriously by society at large (Britney Spears, etc.) and they are right. Marriage is a shell of it's former self. People get married for the heck of it, knowing they can get a divorce the next day. People get divorced because they are arguing, instead of working through it. So why (the argument goes) shouldn't gay people be able to have it since straight people don't take it seriously? Straight people do need to take it seriously to show that it means something. Repealing the no fault divorce is a start.