I would absolutely report it. You have nowhere else to go, they disturbed your rest, and it's just totally unprofessional. How do they expect to garner respect from passengers with that kind of behavior? I would expect better than this. And if they were NOT in the crew rest area, wouldn't that mean they were "on the clock"?
My husband and I once had to witness two passengers going at it in first class, that was bad enough (and tacky), but they weren't working. It was tasteless, but in that case, nothing to complain about.