The core case of those opposing marriage equality or touting the irrelevant study by Mark Regnerus is based on the idea that society can and does pre-emptively judge groups of people by their parenting skills and adjusts their right to marry or have kids accordingly. But that, mercifully, has never happened. Until gay parents came along:
There is no basis in the recent history of American social policy for testing the parenting skills of a class of citizens before we grant them permission to parent — or to marry. Given all the research on the hardships of children raised by single parents, there is still no movement to preemptively remove kids from broken homes after every divorce or to ban single people from having kids; such policies would be patently inhumane and unenforceable. Growing up in poverty increases the risk of a wide range of social and psychological ills, yet since the craze for eugenics died down, no one is proposing banning poor people from marriage or child rearing. And some ethnic and racial groups are statistically less likely to get or stay married, yet there is no ethnic litmus test for marriage or parenting — only a gay one.