Justice Alito made an error similar to Justice Scalia’s during oral arguments regarding Prop 8:
Traditional marriage has been around for thousands of years. Same-sex marriage is very new. I think it was first adopted in the Netherlands in 2000. So there isn’t a lot of data about its effect. And it may turn out to be a — a good thing; it may turn out not to be a good thing, as the supporters of Proposition 8 apparently believe. But you want us to step in and render a decision based on an assessment of the effects of this institution which is newer than cell phones or the Internet?
Once again, he is saying that it may be a good thing or a bad thing, neglecting to acknowledge to possibility that it may just be a thing, with neither positive nor negative effects on society as a whole. Again, this skews the probabilities.
But there are some other issues with this argument. In the first place, this is an argument against trying anything new, ever. It could have been used — indeed, probably was — to argue against extending voting rights of women, or to African Americans. It could have been used to argue against experimenting with democracy in Europe (thousands of years of traditional monarchy, after all!) Lots of advances in civil rights are made without substantial data on their effects: indeed, almost by definition, there must at some point be a country which makes said advances first, with literally no concrete data on effect.
Moreover, as Alito notes, straight marriage has been around for thousands of years, long enough that we don’t have access to any relevant control society without straight marriage to compare against. Thus, there is basically as little data regarding the beneficial, negative, or other effects of state-sanctioned straight marriage on our society as there is gay marriage.