I don’t disagree with what he says – but there are two issues here. First is the principle of gay marriage itself. To argue that society held one view for thousands of years and then changed its view isn’t, by itself, convincing. There have been rapid societal changes on many issues; racism, slavery, women voting, etc.
Then there are the tactics that some people have pursued in achieving their objectives – in this case gay marriage. They have been very illiberal and very ugly. To my mind that would be the only argument against allowing gay marriage – to deny illiberal thugs a moral victory. That too, however, is also illiberal. To deny many people the right to pursue their own happiness simply because some other people are illiberal is wrong.