Santorum getting the Pubby nod is the only thing I can imagine that would make me vote for Obama, but that will do it. Although discrimination against gays offends me on principle, it isn't one of my hot button issues. I have no gay relatives or close friends, so it doesn't really affect me personally. However, the idea of empowering a social crusader whilst our country is falling apart is about the worst thing I can imagine us doing. Santorum is as big government as is Obama, and while his preferences would probably affect me personally less than Obama's, a war of personal liberty is even more distasteful to me than a war on achievers.
My voting matrix is pretty simple.
Romney = Romney
Gingrich = Johnson
Santorum = Obama
Although I usually vote Libertarian, doing so is not an acceptable response to nominating Santorum.