I think we can all agree that, from a risk analysis perspective, global warming and gay marriage share a lot of characteristics. No, no, no— Not that they are comparatively risky— rather that, from a public policy perspective, most people are deeply unsure about just how risky they are.
Start with global warming. Most of the reasons to definitely spend at least some money now on the problem is because of the possibility that global warming could shift us into a new bad climate equilibrium. Now that probability is, best guess, somewhere between really small and maybe 7-8%, depending on who you talk to and whether they are hugging a tree at the time. Since we don’t know the real probability, we have to account for the range of guesses. For example, if half the people think the probability of a new ice age is .1% and half of them think it is 5%, well you can average those two and come up with a best guess of about 2.55%. It is, at least, somewhere to start. Based on that guess, one would, as a matter of public policy, be willing to incur some costs today to avoid a possibly very bad outcome, and how much cost you incur depends very much on the probability you assign to these very bad outcomes. Though less important in this case, you’d also factor in guesses about the probability of a mild good or mild bad outcome.
OK, so there is an even greater dispersion across people about the costs and benefits of gay marriage, but most people probably agree that continuing to ban gay marriage imposes at least some subjective costs on a subset of the population. And by subjective, I mean that the group perceives it as a real cost, even if some other people don’t, so we should factor that cost in. Unfortunately, if you think the climatology research is guess-work, it doesn’t hold a candle to the research on the effects of redefining marriage. We really really really don’t know what effect that will have on society.
Some people think gay marriage will be fine, a few think it will be an improvement for everyone, I’d guess most people think there is some probability that it would be noticeably worse and some probability that it would be no big deal. We might do the same thing we did for global warming, average the views and come up with a best guess of the probability that legalizing gay marriage has large negative outcomes for the society as a whole, as well as guesses about the probability that it had no effect or a beneficial effect. It’s still a guess, that’s all we’ve got, but it is something we could then compare to the costs imposed on those who perceive substantial costs to themselves of not being able to marry people of the same sex. We’d be guessing at those too, by the way. If the expected costs of banning gay marriage outweigh the expected costs of allowing it (meaning you take account of the best guess probability of each of the outcomes you can imagine from the two courses of action), then you drop the ban. Otherwise you keep it. You could well be wrong, but you’ve done the best you can.
At this point, Pareto types would hop in to talk about how those who bear the costs should be reimbursed for their losses from those who gain. Regardless, in both global warming and gay marriage we are trying to weigh the probabilities for outcomes about which we have precious little information. There is some probability that global warming has very bad outcomes, and there is some probability that gay marriage has (completely different) bad outcomes. On the other hand, both of them might be net gains– we lack assurance for either one, nor is scientific inquiry likely to answer the question anytime soon. So we weigh definite costs against probabilistic future gains or losses. And we probably want to compute those probabilities based on some kind of averaging across the population (which is essentially what democracy tries to do).
Based on that, it looks like the best guess is that most Americans think the expected costs of gay marriage are too high to bear, as are the costs of unfettered carbon dioxide emissions.
Of course, God knows the answer in either case. Thus one approach to assigning these probabilities personally is to ditch the population average (which is likely to be better than your own completely subjective guess, but still pretty lousy) and follow the prophet as your best bet of what God thinks. Gay marriage has been pretty well covered. If President Monson says anything about global warming, I’m all ears.