The question brings to mind the "noble lie" found in Plato. In Book III of Plato's Republic Plato has Socrates as saying,
Again, truth should be highly valued; if, as we were saying, a lie is useless to the gods, and useful only as a medicine to men, then the use of such medicines should be restricted to physicians; private individuals have no business with them....
Then if any one at all is to have the privilege of lying, the rulers of the State should be the persons; and they, in their dealings either with enemies or with their own citizens, may be allowed to lie for the public good. But nobody else should meddle with anything of the kind.
There is a worry raised that the guardians of the city--the city's protectors--might begin to love philosophy too much and not do their job (which, again, is to protect the city). Plato toys with the idea of telling them myths about their origin with the goal of inculcating them to love and protect their city. In Plato's utopia, everyone must perform his function well for the city to be virtuous, and thus the guardians must play their part while the philosopher king (or kings) rule. Plato's ideal society is, thus, an enlightened despotism.
Plato, of course, had bad (but intelligent) things to say about democracy. In the ideal society, it is ruled by the wise few, and sometimes for the good of the polis the ruler must use lies to keep the society functioning as a unit. So on a surface reading of Plato, he thinks that there are (or could be) "noble lies." As an ideal community, Plato's isn't bad. Where I would take issue with Plato is with some of his views on human nature. Given my Christianity-influenced view of human nature, practically speaking, I favor Representative Democracies (though I shall not defend that view here). So even if noble lies are permissible in a society such as Plato describes, that does not entail they are permissible in our society. (Things are more complicated than I make them out to be here as far as interpreting Plato goes. The noble lie passage is fascinating, I am leaving a lot of good material out, and I hope to think more about it in the future).
Contrary to Plato there have been some who have thought that it is never permissible to lie, period. Kant and Aquinas come to mind. For Kant, lying violates the categorical imperative; lying can't be willed as a universal law and lying is treating oneself and the one lied to as a mere means and not an end. For Aquinas, lying is inconsistent with Eudaimonia--perfect happiness--which consists in part of perfect friendships, such friendships being incompatible with lying.
That all lying is impermissible is a hard pill to swallow. Is it really morally wrong to lie about Jews you are hiding to the Nazi Storm Troopers inquiring at your door? Is it wrong to lie to a murderer about the whereabouts of your children?
I don't have a settled view on the matter but I am inclined to think that it is only permissible to lie in cases where someone has done something so egregious that they have given up their right to be told the truth. But it is a difficult task to clearly spell out the conditions under which this occurs. For instance, I am inclined to think that a murderer seeking to kill my children has no right to be told the truth of their whereabouts, and I therefore have no correlative duty to tell him the truth. Moreover, they have no right against me not to lie. I am permitted to lie. Nonetheless, someone who murders does not thereby lose all rights to be told the truth. The murderer has a right to be dealt with honestly by his attorney, not only has a legal right, but a moral one. Matters are complicated. (For some excellent thoughts on lying (and against lying) see here.)
Now to the immediate case at hand and Obama's 1 in 5 lie. I would start by questioning whether this lie does rape victims any good. Is "signing the pledge" going to decrease the number of rape victims? I doubt it (though it probably will make some "slacktivists" feel good about themselves). (Furthermore, why not tell the truth about the statistics and ask people to sign the pledge? Aren't there enough rapes already without fabricating the results upward to be concerned about there being too many rapes?) Lying might certainly do some good for those who make a living off of gender baiting ("war on women" types), but to my mind that certainly is not a good justification for the lie.
It is not a good justification because the American people (when did we stop saying "Americans"?) have a right to their President being honest with them and telling them the honest truth. To lie to them is a violation of trust. To lie to them is to disrespect them--to devalue their worth as humans and fellow citizens; it is to treat them with underworth, with less worth and dignity than they have. If Americans have such a right, a moral right not to be lied to--and I hold that they do--then they have a claim against the President to his not lying no matter how much of an increase in life-goods the lie might bring about. To reject the view, is to reject either that Americans have such a right or that rights are trumps to life-goods.