...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.
Blaming lingering Christian memes for the illegality of gay marriage doesn't seem right to me, because almost all countries that currently allow it are predominantly Christian or Post-Christian. Are there any countries that allow gay marriage that don't have a longish history of Christianity?
The modern West treats marriage as being primarily about romantic love, which is an idea not shared by earlier cultures. A culture which does not see romantic love as the essential component of marriage would be unlikely to come up with the idea of gay marriage. There may be some convoluted connection between Christianity and the Western ideal of love-based marriage, but it seems likely that if there were a culture that had the same overriding love-marriage association without any religious objections to homosexuality, that culture would endorse gay marriage.
Suppose I told you that I knew for a fact that the following statements were true:
You’d think I was crazy, right?
Now suppose it were the year 1901, and you had to choose between believing those statements I have just offered, and believing statements like the following:
Based on a comment of Robin Hanson’s: “I wonder if one could describe in enough detail a fictional story of an alternative reality, a reality that our ancestors could not distinguish from the truth, in order to make it very clear how surprising the truth turned out to be.”1
1Source: http://lesswrong.com/lw/j0/making_history_available/ewg.