A good summary of the traditional marriage position can be shown by the two following points:
- (1) Marriage is a privilege not a right. That some have abused it does not make it something all should have.
- (2) Marriage is intended to be a representational microcosm of society in that a representative of each half of humanity is represented in the relationship.
As a friend of mine, Thom Schultz, has once written:
The interaction of that relationship (when done rightly) and the learning that goes on in it furthers culture by showing the couple and by effect, the culture how to be more fully human. This is particularly true in the growth and development of a child. When either gender is not represented, the child suffers for lack of exposure to that perspective. This suffering loss of perspective, which limits the child’s understanding of what it means to be human, also damages the society at large. Male and female both bring important perspective to the child and without it, the child is incomplete.
This understanding can be seen by a variety of advocates: Bobby Lopez (himself raised by two mothers) – see his article Growing Up With Two Moms: The Untold Children’s View here; Doug Mainwaring (himself a homosexual) and his article, I’m Gay and I Oppose Same-Sex Marriage can be found here; Ryan T. Anderson, Ana Samuel, Sherif Girgis, Robert P. George, and of course many others. In fact, according to one article by Sherif Girgis:
Marriage is a human good with its own structure, like knowledge or friendship. The present debate is not a debate about whom to let marry, but about what marriage (the human good that the law has reasons to track) really is. Two answers compete for legal enshrinement.
The first, driving the push for same-sex marriage, is that a certain emotional intimacy makes a marriage. But as our book shows, this answer can’t coherently distinguish marriage from companionship, an obviously broader category. So it gets marriage (the human good) wrong.
The second view of marriage begins from basics. Any voluntary form of community involves common action; it unites people toward common ends in the context of commitment. And in these respects, what sets marital community apart is its comprehensiveness: in (1) how it unites people, (2) what it unites them with respect to, and (3) how extensive a commitment it demands. 
The stability of marriage, so understood, best ensures that children will know the committed love of those whose union brought them forth. This gives them the best shot at becoming healthy and happy people, which affects every other social good. That is why every society with the merest ambition to thrive has socially regulated male-female sexual bonds: to shore up the stabilizing norms of marriage, on which social order rests. 
To finish, there are a couple things to make clear about the conversation: (1) this is a clearly secular argument against redefining marriage and not a religious one. (2) I have more concern about the redefinition of marriage rather than on who marries. Hence, the conversation should not be centered on a moral debate but rather than a definitional one (that isn’t to say the moral isn’t important or considerable, but that it isn’t the initial issue that should be brought up).
-  Sherif Girgis, Check Your Blind Spot: What Is Marriage? (Public Discourse: Feb. 2013)
-  Ibid.