We can't really talk about marriage as if it's a fixed and unchanging institution, although previously it has always been between a man and a woman (worldwide I think). That would, of course, have been mainly to ensure the reliable legacy of wealth and title to the offspring and the avoidance of cuckoos in the nest. (Nowadays there is DNA testing.) Nevertheless, the institution as such has changed and these changes were probably accepted with great reluctance among the many. Until the Married Women's Property Act, marriage meant a woman gave up all rights to her possessions to her husband who had no accountability about what he did with them. At one time, women could legally be forced into marriage against their will with no way of avoiding it. Husbands could divorce wives, but not vice versa. There have always been changes and modifications to all social activities and contracts. Marriage for gay men and women is just one more, albeit quite a sizeable change and, for many, long overdue.