Here is the relevant text from the transcription of his monologue (emphasis mine):
"Mankind has a long history of branding evildoers, starting with God marking Cain after the death of Abel, right through to Colonial New England, where they slapped a scarlet letter on your chest if you were guilty of illicit sex. Of course, nowadays if you have dangerous sex, they put on AIDS ribbon on your chest and you get to go to the Oscars."
I know this was 1993, when most people in the United States were first starting to learn about HIV/AIDS, but even back then this was not an acceptable thing for a left-leaning celebrity to say. HIV/AIDS shaming like this--whether done in 1993 or 2015--is fallacious victim blaming that also has obvious homophobic, racist, and sexist connotations. And in the video clip, most of the audience seems to be taken aback.
I've found nothing on the internet referencing the joke. If he had said this in more recent years on Real Time--particularly in the pilot episode--or in a tweet, many people would have written think-pieces about it in online publications like Salon and Mother Jones. Here's the clip, with the monologue starting at 2:57: