This argument is a favorite non sequitur of gun control advocates. It really only holds if one regards all weapons as morally equal. It is a bit of a stretch to regard nuclear weapons as the moral equivalent of guns.

A gun is a weapon which can be used (and is almost always used) by one individual against another individual. A nuclear weapon is useless against individuals and is only used against large groups of people for indiscriminate killing. A nation, even one with an aggressive army is never comprised of only aggressors. Using a gun against an invading army is a case of people using a weapon against someone who is actively attempting to kill them. Nuking a city kills innocent people along with guilty, hence its immorality.

A nuclear weapon is one which is impossible to use against only military targets, which is why I don't support them being used. I would never support the legalization of a weapon for the government which is not also legal for the individual. The real question is why would anyone who considers themselves moral, would think that is fine to use a nuclear weapon under any circumstance, be that use by a government or an individual. Why is government exempt from morality simply because it is a group of people acting?

A gun can be used in a just cause, by an individual, to protect the innocent against an aggressor without violating the rights of anyone. A nuclear weapon cannot be used in such a limited manner.

Droidkevin5 reminded me of the difference between strategic nuclear weapons, which are the only kind ever actually used in a war, and tactical nuclear weapons, low yield devices used specifically against military targets with a small blast radius and low/no impact on civilian areas. Those might be usable in a moral manner. But what we normally know as the bomb is a weapon which can only be used in a murderous manner in a war between nations.