I agree with the statements of fact but not with the inference drawn from them. While Jiro's argument is poorly expressed, I think it is reasonable to say that opposition to homosexuality would not have been the default stance of the cultures of or derived from Europe if not for Christianity being the dominant religion in previous years. While the Communists rejected religion, they did not fully update on this rejection, but rather continued in many of the beliefs that religion had caused to be part of their culture.

I am not sure that "the atheists actually thought gay marriage was a sane idea but didn't say so for fear of how they'd look to their religious neighbors" was Jiro's position, but I think that it is a straw man.