They've been pursuing nuclear weapons for decades, and bit by bit getting closer. A nuclear armed Iran would result in the rest of the middle east - most of which considers Iran a serious enemy - to pursue their own nuclear weapons program.
There is nothing far-fetched about countering Iran's nuclear ambitions: they have been actively and blatantly pursuing it for decades.
A nuclear armed Iran would quickly lead much of the middle east to pursue their own nuclear weapons programs to counter Iran: in that part of the world, Iran is considered a very serious enemy.