You can clearly see from the pictures that it is him -- the face is obscured, but you can see his beard, and it IS his beard, and the "neck" thing is him, too. And there's enough obvious attempt to 'disguise' himself on the site -- he only identifies himself as "andy" -- that the sex-cruising posting certainly seems to be genuinely Andrew Sullivan, ie, and not a put-up job by someone trying to smear him.
It's the real deal. Sullivan himself confirmed a few years ago that the ads were him.