"The law is a great teacher, and same sex marriage will teach future generations that marriage is not about children but about coupling. When marriage becomes nothing more than coupling, fewer people will get married to have children.
People will still have children, of course, but many more of them out-of wedlock. That’s a disaster for everyone. Children will be hurt because illegitimate parents (there are no illegitimate children) often never form a family, and those that “shack up” break up at a rate two to three times that of married parents. Society will be hurt because illegitimacy starts a chain of negative effects that fall like dominoes—illegitimacy leads to poverty, crime, and higher welfare costs which lead to bigger government, higher taxes, and a slower economy."
The opening argument is a little weak. Fewer people will get married to have children? I hope people are getting married for more than just simply having children. Marriage isn't simply a blind attempt to reproduce, as far as I know. I was under the impression that, according to the strict conservative that Turek seems to be, marriage was an expression of love between two people. Once those two married people decided to have children after marriage, they could then cross that bridge; at the time of legal and religious marriage, though, future children aren't meant to be the primary motivation.
Also, "The law is a great teacher, and same sex marriage will teach future generations that marriage is not about children but about coupling." So, because gay people are getting married, I won't want to have children in my marriage? Right... uh... I'm doubting it.
Gay marriage, in fact, will be quite a positive. Their legal bond will open up a whole new venue for adoption (already possible, but isn't often pursued due to legal complications, etc.). All the ideas pursued in the second quoted paragraph will actually go in the other direction.