I know this might not be too helpful as an explanation, but perhaps Riley was screening a film based on a book he published, making sure that it met his standards for his part in the copyright deal. That's a guess, anyway.

Interesting! I don't know if this helps very much, but the subtitles show a little bit that might help support your softcore-type-film theory. Here they are, from Netflix:

[Light Jazz Playing]
Riley, smoking: Yeah
(Columbo enters)
Riley: Ha ha ha
Columbo: Excuse me, Mr. Greenleaf?
(Riley, annoyed, shushes him, and resumes his cackling at the film)

There's also the fact that Columbo, usually interested in everything going on around him, studiously avoids looking at the film while he's in the screening room, until Riley tells "Andy" to turn off the film.

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So, in favor: Riley is smoking (cigarettes formerly having been used to allude to more than one type of "satisfaction"), cackling in a lurid manner, and annoyed at the interruption... and Columbo won't look at the screen.

Against: "Andy" was there the whole time (?!). Also, Riley doesn't hurry to shut it off, and one would think he'd be a little more discreet with it, if it was tawdry.

See the 'Greenleaf Publishing' thread.

thanks, that's exactly how I see it, the same factors -- Andy being there, Columbo's non-reaction, etc.