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.
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.