In my hall of fame of disco records used without license in gay porn:

One of the farmer/trucker-themed Al Parker movies used country singer Bill Anderson's "I Cant Wait Any Longer, which is on a very rare MCA promo 12-inch, which plays behind an extended sex scene in the back of a pick-up truck.You may recall the spoken lines from the record:
"Patience has never been one of my virtues;
When I want something, I want it NOW."

"Action '78" by Erotic Drum Band is used twice in a Kip Noll film where he and a much taller dark-haired man dance briefly to the music, then Kip gets fucked on a pool table. After the money shot, the music and clothes magically reappear and they resume dancing and fade to black.

Saint Tropez' "One More Minute" plays behind a hard-pounding sex scene on a pre-condom VHS compilation released in the 1990s which was mainly rimjob-themed. The expression on the bottom's face is priceless.