1. Who 'legalized' heterosexual marriages?<quoted text>
I will add one thing to my response above... If incestuous marriages "might" result from allowing same-gender marriage; why is it that incestuous marriages didn't spring forth from the legalization of heterosexual marriages?
2. Because at it's most basic essence, marriage is a cross cultural constraint on evolutionary mating behavior. For the sake of children. Incest doesn't qualify, which means ss couples don't qualify even less.