Don't Start A War if You Can't Take a Joke

Nuclear weapons do not require a "Moral Revolution". The Tokyo firebombing killed more people than Fat Man or Little Boy. It doesn't matter whether it takes 1 plane or 100 to kill a city - the morality lies not in the how, but in the what and why.
It would have, in fact, been immoral to NOT nuke Imperial Japan. All the other options were far worse:

1. Allied land invasion, with Soviet support. Millions of Allied soldiers and Japanese soldiers and civilians die, with many more injured. The Soviets (who were pretty evil themselves) probably would have tried to use the opportunity to grab as much land as possible.
2. Allied land invasion, without Soviet support. Same results as above, only instead of taking chunks of Japan, the Soviets would have tried doing it in Manchuria, Mongolia, and Korea, and with a much lighter death toll for the Soviets.
3. Starve them out. Blockade all Japanese ports, bomb food stockpiles and production. Would have resulted in millions of Japanese dead, serious national health issues. and an utterly devastated nation.

Not to mention that if anyone ever deserved to get nuked it was Imperial Japan. The Rape of Nanking, Unit 731, the Bataan Death March, the list goes on and on. We bombed some of the worst mass murderers in history, to save MILLIONS of lives. Moral Revolution my ass.