There are some confusing aspects of Lexi's play because it's unclear if the scene is happening in Lexi's play or outside of it.
The perfect example of this would be the scene that begins with Lexi's voiceover explaining that Nate (Jacob Elordi) and Maddy's relationship was her and Cassie's first idea of real love. At first the characters are played by Nate and Maddy's look-alikes, but suddenly, the camera pans to Nate in the audience, and then we're watching the real Nate watch Maddy undress, only for her to change into Jules, who approaches Nate and hugs him. Then Nate pushes a girl down on a bed and rips off her tights, but that girl is now Cassie. Finally, Cal replaces Nate as the man who's looming over the girl on the bed, but when he leans down, it's now Nate who's laying face down on the bed.
It's unclear if any of this actually transpired in Lexi's play, but chances are it didn't, since it's hard to believe Maddy or Cassie would have revealed this much to Lexi, no matter how observant she is. It's more likely that, in cases like this, the characters in the audience are processing what's happening in the play in their own minds and filling in the blanks.
If the scene did take place in Nate's psyche, it begs the question: what does it mean about Nate and Cal? Some fans believe the scene is hinting at the possibility that Nate was sexually abused by Cal as a child, but other fans believe that the scene is Nate processing the emotional and psychological effects of having watched Cal's porn collection.