It really matters that we recognise this as a new social institution. As a Christian I would argue that being a man or a woman is not incidental to the human relations a person may engage in, but formative of them.
In Christian understanding the human meaning of sexual difference is rooted in the good gift of God in creation. The male-femaleness of the human race is given to us, it is where we are placed, in common with the whole human race in every generation, and our role is to be thankful for it and to understand how it helps us to live the human lives we are given.
This task of appreciating our sexual difference weighs equally on married and unmarried, on gay and straight, on children and adults – on all who have the gift of being human.
Christians, in common with Jews and Muslims, understand marriage as essentially representative of this good gift of sexual difference. This understanding flows from an undivided and unbroken tradition which has helped to define the unity of the human race, uniting nations, religions, cultural traditions and periods of history. – See more at: http://www.archbishopofyork.org/articles.php/2919/same-sex-marriage-bill-committee-stage#sthash.YGO8QtJO.dpuf