It can be both. Natural marriage serves to promote the earthly good of husband, wife and children, and therefore of society. Sacramental marriage is ordered toward the eternal salvation of all of the members of the family.
I concur in certain respects: marriage is intended to be a dim reflection of the union between Christ and His Church (which makes the recognition of homosexual unions even more distasteful, imho), so it definitely has distinctly Christian qualities. Those distinctly New Testament qualities are vital for the believer, but not essential for this debate.