As a strongly secular libertarian and conservative, I normally agree with Derbyshire; in particular I admire his writing on China. But I disagree with his secular case against gay marriage.
In an ideal libertarian world, the government would have no power to say anything about who you choose to spend your life with - it's a contract between two people, and between them only. Not coincidentally, Ayn Rand revivalists, it's this relationship, among the most basic of human bonds, that is at the center of her first work of fiction Anthem, which later inspired works as far ranging as Rush albums to Star Trek episodes.
Miss California 2009 is among those making a faith-based argument against other people's right to marry. She's honest, but she's wrong to do so. Whether or not you are religious, if you accept our Constitution as the law of the land, that's a non-starter. When government or other citizens tell me what to do, they damn well better have a real-world, proveable, factual rationale for doing so before I even listen. That's what Jefferson meant when he talked about not caring what his neighbor did, so long as his neighbor didn't break his leg or pick his pocket. So if someone tells me I'm not allow to drink alcohol or eat pork, I ask what's their reason? If it's the Qu'ran, thanks but no thanks, and if you need me I'll be at the ballgame with a beer and a hot dog (and calling them a mush-head, a character defect which conservatives don't suffer gladly). Same for you, Miss California: you have every right to speak your mind, but none to tell me who to marry, so long as you can't offer any factual, concrete reasons.
Derbyshire does try to offer factual, concrete reasons against gay marriage, and while I disagree, this is at least the right way to go about making your argument. What I will say is this: while I support marriage equality, the current political climate is so full of reflexive partisan rhetoric that anyone making an honest, factual inquiry about the effects of gay marriage on civilization is likely to be branded a homophobe. (See how I put a disclaimer immediately in front of that statement?) But it's worth asking these questions. It's naive to automatically assume that there will be no impacts from such a seismic shift in the makeup of families.
At the same time - and this is critical - if you're a conservative, the burden of proof is always on those who would take a freedom away. Until you can show the facts of exactly how gay marriage hurts other people, it should be allowed. The similarity of anti-gay marriage arguments to anti-interracial marriage arguments makes me suspicious that there are no such facts under the histrionics. There have been studies, and kids raised by same-sex partners are not any more likely to themselves be gay, and these kids do just as well as kids from traditional marriages. This isn't some lobbying organization spinning data to suit their purposes - that link goes to the American Academy of Pediatrics. Doctors.
As a conservative, I ask that we all separate our personal faith from our political demands on others. Using the government to deny people freedom without evidence is not a good habit to get into, especially for a conservative. When you have facts, show them to us. Until then, support marriage equality.