I would contest that assertion- I see marriage as a social and legal institution first and foremost, and the religious associations simply reflect the human tendency to tie religion to any major event in their lives. Remember, most of these institutions, in their basic forms, date back to a pre-Abrahamic era, when the line between religious and secular life was far less well defined, and the modern concept of secular life simply didn't really exist, so the fact that Christianity later usurped paganism as the associated religion does not at all imply that it also usurped the more ancient secular aspects of marriage.