There is no guarantee that we'll nuke Iran just because they nuked Israel. In fact, it's very likely that we wouldn't do so. Iran's government knows that as well as anyone.
Besides, their "defensive" capability is merely cover for them to increase their regional presence in terms of more overt support of groups in other countries. They don't really want to nuke Israel. They just want to increase their influence in Iraq or Syria or Lebanon. If Israel is likely to have a problem, that will come about due to more overt activities to support proxies like Hamas.
The real threat of Iran with nukes isn't nuclear war, per se. It's that they get a free hand to increase their interference in the region and that's very dangerous. Say what you will about US invasions, but no one seriously believed that we were there to stay. Iran is right next door to a lot of countries that don't want to have to deal with a nuclear Iran which would be able to hold off the US, because the US is what is guaranteeing the balance in the region between Muslim states that hate each other's guts. A nuclear Iran could cause a country like Saudi Arabia to obtain its own nuclear weapons because the threat of a US invasion in support of the status quo is lessened.
The reality is that we're not just trying to keep nukes out of the hands of the Iranians, we're trying to make sure that the rest of the ME doesn't enter an arms race which puts nukes in the hands of other countries.