When the law of the land and my neighbors' practice of marriage reinforces that promises and vows don't matter, it's more likely that I'm willing to think and act the same way, buying into a no-fault divorce culture.
When the law of the land and my neighbors' practice of marriage trains me to view children as a mere afterthought to marriage (and certainly not part of the essence of marriage), it's more likely that I'll see having and raising children as one option among the smorgasbord of life choices with which I'm faced. I might even tend to see my childrens' existence (or lack thereof) as something that is ultimately geared toward making me happy and fulfilled.
When the law of the land and my neighbors' practice of marriage encourages me to see marriage as an avenue of individual self-expression and self-growth, to be abandoned if my spouse fails to play a role in that process, it's more likely that I'm willing to think and act the same way.
The atheist W. K. Clifford summarizes well the point that none of us is an independent thinker or actor:
"And no one man's belief is in any case a private matter which concerns himself alone. Our lives are guided by that general conception of the course of things which has been created by society for social purposes. Our words, our phrases, our forms and processes and modes of thought, are common property, fashioned and perfected from age to age; an heirloom which every succeeding generation inherits as a precious deposit and a sacred trust to be handed on to the next one, not unchanged but enlarged and purified, with some clear marks of its proper handiwork. Into this, for good or ill, is woven every belief of every man who has speech of his fellows. An awful privilege, and an awful responsibility, that we should help to create the world in which posterity will live....
[N]o belief held by one man, however seemingly trivial the belief, and however obscure the believer, is ever actually insignificant or without its effect on the fate of mankind."
In the end, the kind of radical individualism expressed in the sentiment "my straight marriage can't be affected by anyone else's view or practice of marriage" is, ironically, one of the key indicators of an individualism that can't help but corrode the institution of marriage. Please note: I am not saying gay marriage is going to cause the erosion of Christian marriages. I am saying that the widespread cultural acceptance of gay marriage is a symptom of how our culture already thinks about and practices marriage. Thus, rather than congratulating ourselves for not affirming gay marriage, Christians who affirm Scripture's authority need to subject their own views and practices of marriage to sharper scrutiny. In a world that sees each individual as sole captain in a world of their own making, Christians ought to be humble enough to recognize that there is nothing we have not received--including our views and practices of marriage--from somewhere. The question is: where?