I haven't followed Sullivan's blog in months, but I would guess his position on this is based entirely on his commitment to the "Sarah Palin faked her last pregnancy" conspiracy theory. He's staked his reputation on this, with the logic of "If I'm wrong, Palin should just disclose her son's birth certificate and I'll shut up," so rhetorically he's backed himself into a corner where every public figure has to prove the crazies wrong.