So, I went and checked the definition of "moral realism" to understand why the term "dangerously" would be applied to the idea of being close to supporting it, and failed to find enlightenment. It seems to just mean that there's a correct answer to moral questions, and I can't understand why you would be here arguing about a moral question in the first place if you thought there was no answer. The sequence post The Meaning of Right seems to say "capable of being true" is a desirable and actual property of metaethics. So I'm no closer to understanding where you're going with this than before.
As to how I determined that opinion, I imagined the overall negative effects of being exterminated or sent to a concentration camp, imagined the fleeting sense of happiness in knowing someone I hate is suffering pain, and then did the moral equivalent of estimating how many grains of rice one could pile up on a football field (i.e. made a guess). This is just my current best algorithm though, I make no claims of it being the ultimate moral test process.
I hope you can understand that I don't claim to have no idea about morality in general, just about the exact number of grains of rice on a football field. Especially since I don't know the size of the grains of rice or the code of football either.
Moral realism claims that:
[True ethical propositions] are made true by objective features of the world, independent of subjective opinion.
Moral realists have spilled oceans of ink justifying that claim. One common argument invents new meanings for the word "true" ("it's not true the way physical fact, or inductive physical law, or mathematical theorems are true, but it's still true! How do you know there aren't more kinds of truth-ness in the world?") They commit, in my experience, a multitude of sins - of epistemology, rationality,...
This is based on a discussion in #lesswrong a few months back, and I am not sure how to resolve it.
Setup: suppose the world is populated by two groups of people, one just wants to be left alone (labeled Jews), the other group hates the first one with passion and want them dead (labeled Nazis). The second group is otherwise just as "good" as the first one (loves their relatives, their country and is known to be in general quite rational). They just can't help but hate the other guys (this condition is to forestall the objections like "Nazis ought to change their terminal values"). Maybe the shape of Jewish noses just creeps the hell out of them, or something. Let's just assume, for the sake of argument, that there is no changing that hatred.
Is it rational to exterminate the Jews to improve the Nazi's quality of life? Well, this seems like a silly question. Of course not! Now, what if there are many more Nazis than Jews? Is there a number large enough where exterminating Jews would be a net positive utility for the world? Umm... Not sure... I'd like to think that probably not, human life is sacred! What if some day their society invents immortality, then every death is like an extremely large (infinite?) negative utility!
Fine then, not exterminating. Just send them all to concentration camps, where they will suffer in misery and probably have a shorter lifespan than they would otherwise. This is not an ideal solutions from the Nazi point of view, but it makes them feel a little bit better. And now the utilities are unquestionably comparable, so if there are billions of Nazis and only a handful of Jews, the overall suffering decreases when the Jews are sent to the camps.
This logic is completely analogous to that in the dust specks vs torture discussions, only my "little XML labels", to quote Eliezer, make it more emotionally charged. Thus, if you are a utilitarian anti-specker, you ought to decide that, barring changing Nazi's terminal value of hating Jews, the rational behavior is to herd the Jews into concentration camps, or possibly even exterminate them, provided there are enough Nazi's in the world who benefit from it.
This is quite a repugnant conclusion, and I don't see a way of fixing it the way the original one is fixed (to paraphrase Eliezer, "only lives worth celebrating are worth creating").
EDIT: Thanks to CronoDAS for pointing out that this is known as the 1000 Sadists problem. Once I had this term, I found that lukeprog has mentioned it on his old blog.