Deliberate mistake: When Betty walks away from the kissing booth it is clear that she has spike-like heels on her boots. When she steps on to the "moon" surface the spike heels are gone. Her spikes are on up to the point where they enter the room. I am pretty sure the crew did not want to puncture the bladder and have water all over the set. (01:07:25 - 01:09:35)
Continuity mistake: In the scene where Lamar is throwing the javelin, you can see the scoreboard for the games. The Tri-Lambs currently lead the games with 10 points (which was after the tricycle race), but at that part of the movie, the Alpha Betas currently led and had at least 40 points. (01:01:25 - 01:06:20)
Answer: First, Lewis and Betty are not "having sex" - he is raping her, and the writer and director later described it as "not excusable" (https://decider.com/2019/07/26/revenge-of-the-nerds-rape-scene-regret/). Second, the song you hear is "Right Time for Love" by Jill Michaels and Pat Robinson. It was not written for the film.