None. To the contrary, gay marriage actually strengthens society.
That is not just false, it is hysterically false.
Marriage is the joining of one man and one woman.
Marriage is such, because that is how nature designed the human species, with two distinct, but complimenting genders.
Marriage provides for the traits respective to each gender to nurture and train children though the distinct perspectives common to each. By lowering the standards of marriage, the Ideological Left has long since injured the institution of marriage... resulting in exponentially more children being born into single parent homes, encouraging divorce, rendering two gender families into one gender families...
The evidence is conclusive that children living in homes where one gender is represented children tend to grow up absent sufficient training, thus lacking the discipline to provide for them the means to focus cognitively, thus producing the epidemic of drug addles youths through the ill-advised treatment of the psychological alphabet soup maladies; ADD, ADHD, ODD and so on and so forth of fictitious 'disorders' which has lead generations of children who have failed to develop into sound, well balanced adults.
Adding further devolution of the marriage standard is only going to promote more perverse iterations of poorly founded children with little means to function independently and exponential growth in children who develop disorders wherein they identify themselves as homosexual or some other form of sexual and maturation abnormality.