How didn't Tommy know?
Renton asks to borrow "100 Greatest Goals", Tommy says yes. When he and Lizzy copulate, they watch their video. However, the video that comes on is not their sex tape, but "100 Greatest Goals". We know that's the video, as we saw Rention swapping those tapes around and the footage that comes on the TV in front of Tommy very much looks and sounds like something from "100 Greatest Goals".
Now, if you were Tommy, would your immediate thought not be Renton? He asked to borrow that video, and when you put on your sex tape, that's the video that comes on instead... Seems very suspect. I don't feel like it's something that Renton could just casually shake off either. If Tommy actually quizzed him I think it would get obvious quick, especially if he asked Renton to give him back the tape he borrowed.
Is this not a plothole? Am I missing something?