Looking for an oldie but goodie short flick

As early as perhaps 1979 (but possibly as late as 1981), I remember this 15 minute or so flick that I somehow got to see. Perhaps this was a complete short movie in and of itself, but it could have been just one part of a longer movie. Ever since the internet has facilitates the finding & viewing of movies (especially porno), I've been wanting to locate this. I will describe the "plot", which may ring a bell.

The flick starts with some couple (not the main actor & actress) just having regular missionary sex (although she seemed to be sitting to some extent, not lying down totally), with some evidently prudish woman (the main actress) somehow passing by and noticing this activity. She proceeds to complain about this to some man (the main actor) - in which somehow they were in a room with a bed (of course,) He asks her if the woman who was seen having sex was wearing a shirt, to which she answers no, and he tells her that she needs to take off her shirt. This goes on for about half a minute with the other articles of clothing until she is buck naked. He then asks her what the man was wearing, which gets her to smile and then the camera speeds up with some funny music as she helps him remove his clothes, ending up with a shot of his long erect penis, with a "boing" sound. Then they evidently proceed to recreate the positions as doggie style and a few others. The final position is him sitting on a chair with her riding him (facing away from him), with her getting off of him, and then sitting down in back of the penis where she rubs it for a second of two until ejaculation.

So that's it - I have no idea who the actors were. Obviously this was an enjoyable little movie for me, as it's been over 30 years, and I can recall it so well.