They would have, had they known in time. That's why Israel kept it secret from the U.S. Read this recent Politico story that draws on newly available documents to explain what happened. http://www.politico.com/magazine/story/2015/04/israel-nuclear-weapons-117014.html

"Cohen’s new statement, paired with recently unearthed U.S. government documents, clarifies for the first time this historical puzzle and sheds new light on how Washington missed warning signs about Dimona, how it belatedly discovered the reactor and, later, how it reacted to the finding. "

"In a sense, the secrecy surrounding Dimona was aimed primarily at the United States. Of all the powers, Washington posed the greatest threat. Since the time of the Baruch Plan in 1946, the United States was on the record as an opponent of the spread of nuclear weapons. Moreover, Washington helped create the International Atomic Energy Agency (IAEA) in 1957, the very same year the French-Israeli deal was signed, and since then it had promoted the establishment of an international safeguards system. Should the Dimona secret have been compromised, the United States would have likely exercised pressure on France and Israel either to terminate the project altogether or at least to submit it to international safeguards. "

They did oppose it, when it seemed like there was something that could be done about it. The Kennedy administration was no fan of an Israeli nuclear arsenal — they saw it as destabilizing and dangerous to the region. They confronted the Israelis about Dimona, even forced them to let them inspect it in 1961 to confirm it was non-weapons-relevant like the Israelis claims. The Israelis literally cemented off the weapons-relevant parts of the facility (that were hidden underground) when the American scientists came around to take a look.

The real "acceptance" of the Israeli nuclear program was not hammered out until the Nixon administration. Nixon and Kissinger took a more "realpolitik" approach to the whole thing — the Israelis had the bomb, that fact wasn't going to likely change anytime soon, the US needed a staunch ally in the region, so as long as the Israelis didn't do anything that would be too provocative (like flaunt their weapons or test them or use them), they could live with it, using a fig leaf of "opacity" (don't confirm or deny and the US doesn't have to deal with it).

Avner Cohen's work, summaries of which have been posted at GWU's National Security Archives along with documents, is excellent on this turning point, esp. the way in which the language of what it meant to "introduce" a nuclear weapon into the region served to codify the fig leaf.