Elsewhere, we also refer to epistemic rationality, which is believing true things. In neither case do we say anything about what you should want.

This begs the question against moral realism. If it is in fact true that harming cows is bad, then epistemic rationality demands that we believe that harming cows is bad. Of course, saying that you should believe harming cows is morally wrong is different from saying that you shouldn't choose to harm cows, but the inference from one to the other is pretty plausible. It seems fairly uncontroversial that if one believes that action X is bad, and it is in fact true that action X is bad, then one should not perform action X (ceteris paribus).

I don't agree with davidpearce's framing (that rationality demands that one give equal weight to all perspectives), but I also don't agree with the claim that rationality does not tell us anything about what we should want. Perhaps instrumental rationality doesn't, but epistemic rationality does.