Nuclear weapons being used by any country, even peace keeping Iran in contrast to Iraq would be in place to destroy the world. Nuclear war would be catastrophic and should be avoided at all costs. Often giving nuclear armament to the small countries that are not powerful could be dangerous.
Yes, a nuclear Iran is intolerable, because a nuclear Iran poses great threats to the rest of the world. Iran wants to expand its power, and they are actively working on increasing their nuclear capabilities. It would be safer and would be less costly in economics and human life to intervene now, rather than later on, after the development of weapons.