The burden of persuasion on gay parenting

Reason & Liberty: Shifting the burden of persuasion on gay parenting: "Thus, the burden must be on those who seek to exclude gays from these institutions to justify that exclusion by means of concrete arguments and actual evidence about how gay families produce harm or diminish social utility. To retreat to assertions about tradition or 'morality,' red herrings about incest or polygamy, or cliches about being 'pro-family' is simply to admit you have nothing more rational and persuasive to offer."

Steve Sanders makes a cogent point in observing that we must stop allowing the bigots to walk away from actually defending their perspectives in terms of political science. Along these same lines, where is the argument that says the homosexual undermines society? I know the statement is made often, but where is the argument? Upon what facts is this based?