When people here said they gave a high prior to moral realism (i.e. did not dismiss it as I did), I assumed they were rational about it: that they had some evidence to support such a prior. By now it's pretty clear that this is not the case,

Perhaps they misunderstood what was referred to by "moral realism". The phrase certainly doesn't seem to be very well defined. For example Eliezer does say that there are things that are actually right, and actually wrong. mwengler seems to think this is sufficient to make him a moral realist. You don't. Classic recipe for confusion.