Well, technically, it's not slander. No reputational harm to any individual that can be measured or remedied.
But I'm curious. What is your proposed solution? Countering Leary's routine with other speech, or something else?
Well, it is closer to defamation, if we are at law. But why must we be at law? Community standards are the problem. Neither blacks, nor Jews, no homosexuals may be subjected to such public ridicule. And he sure does not do it for free. He is playing to an audience that laps this stuff up and pays to hear more. In the end it is up to Catholics to shut this guy up. But we are at handicap, because the people who operate this "show" share his sentiments and because the American Church is not unified in the way that it was fifty years ago.