Takashi: I think I have a frush.
Booger: What the fuck is a frush?
Stan Gable: Times are changing, Betty. These nerds are a threat to our way of life.
Betty Childs: If they win the Homecoming Carnival, they'll be able to take over the Greek Council.
Stan Gable: Yeah. And we'll be up to our asses in nerd shit.
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.