one should oppose the use of torture.

It would be trivial for me to construct a hypothetical where torture is unambiguously a good idea. It wouldn't even be hard to make it seem a realistic situation; I might even be able to use a historical example. To call something generally irrational, or to claim that rationality is opposed to a thing, you have to make the argument that in principle it's not possible for this to be either a terminal goal or the only available instrumental goal.