I like this reply, but I feel it doesn't take the next logical step. What kind of considerations could make utilitarianism correct, given that, as you suggest, a good society needs some firmer rules?

So, just suppose for a moment, a bunch of rational human beings (rational, as human beings go) get together and agree to live by some rather rigid rules. They do so with the best available evidence in front of them, after thinking things through as well as could possibly be expected. They tell each other that you shouldn't push the fat man in front of the trolley, and act accordingly.

What possible sense can it make to say that nevertheless, really and truly, the morally right thing to do is push the fat man? What would "morally right" mean in that sentence, when we have already stipulated that pro-social codes of conduct and recognized virtues, rationally agreed to in open honest well-informed discussion, recommend something else? There is nothing else for morality to "really and truly" be about.