It is a very interesting argument–and I have to say, better than any of the other ones I’ve heard offered. So I’ll give you that.
However, to me, this isn’t a reason to oppose something–but instead, I see it as a rule of decision. It is a way to figure out if you oppose something or not–and one that abrogates any need to come up with a reason on your own, or to test it.
So I would point out three issues with it. I’m not even sure they’re fatal—but I find them troubling.
First, I hope you see that this cannot be generalized. Literally, you cannot posit that a majority of the population could reasonably reach a decision using this method, because then (1) either that majority could not decide how to think–because they would look to the majority to find out how to think, but they are the majority (and haven’t decided yet), or (2) if, instead, they look to the majority of those who have an opinion, you risk having a very small minority drive their decision–so, (hypothetically), if 60% of the population used your method, the “majority” who would drive their decision would be 20.1% of the population–a majority of those who held an opinion on the issue). Further, if they choose (2) rather than (1), the more people who endorse your method, the less reasonable their decision is–if more people who use this method, fewer people are left to have their opinions looked to–and a smaller “majority” will control the choice of those using your method. Further, if we contend your method is reasonable, it seems more likely it is that those not using it are unreasonable–such that it compels them to make their decision based on the opinion of a small, and perhaps unreasonable minority.
(2) seems inconsistent with the reason for holding this stance (that you look to the majority)–since the more popular it is, the more it compels you to make your decision based on the stance of a small minority. and (1) is, of course, no use in making a decision. Hence, at the least, this justification cannot, by definition, be the reason a majority opposes same-sex marriage.
Second, it seems to me that the real reason at work here is that held by the majority to whom our voter looks-- (if you assume that the majority holds an opinion for a reason)–just like as if I say I will agree with the next thing you say: is my position then reasonable? It is if the next thing you say is reasonable, and is not if it isn’t.
To put it another way: I would argue that a rule of decision that compels you to follow a certain line of reasoning, or to decide in a certain way, even if that line of reasoning or decision is unreasonable, or just plain wrong, is not in itself reasonable.
So I would either argue (narrowly), that the reasonableness of holding this position must turn on the reasonableness of the justification(s) held by those you look to to make the decision, or (broadly) the method is unreasonable because it involves no examination of those justifications, or the position held by a majority–such that it would not just permit, but compel you to adopt an unreasonable decision or plainly wrong answer if held by the majority.
Third, and relatedly, sometimes the majority is just plain unreasonable. Sometimes they ignore the evidence. Evolution is a good example. (Assuming I recall the statistics correctly) A substantial minority of Americans actively reject evolution, and only a minority in fact “believe” it to be correct. (and we can think of many more examples). Now, you and I both know that evolution is just plain scientific fact.
So, if someone using your proposed method was deciding whether evolution was correct, he would ask whether a majority so believes. Given that the answer to that question is no (since a majority is either “no” or “don’t know”–and hence only a minority would vote “yes”), someone using your method would have to vote “no.” Significantly, that decision will be made independently of, and in fact in spite of the evidence on the issue.
Let us, for example, assume that your decisionmaker was a professor of biology-someone who is familiar with the scientific literature, and the evidence for evolution. If he followed your proposed argument, and voted against evolution, he would be knowingly voting for a position that was inconsistent with all the evidence on the issue. Even if he didn’t know the evidence himself, he would be taking a position whether or not it was factually supported.
And I would contend that it is unreasonable to do so–to use a means of decision that would force you to take a position contrary to compelling evidence, simply because a majority doesn’t understand that evidence, or is unreasonably interpreting it.