Honestly, that's such a boring way to look at the game. Any story, even that of a porn game, relies on it having some kind of conflict. Without it, you cannot have an interesting story. If the game followed your own logic of Jeremy and Lena just sleeping with one another in secret without any further development than that would be extremely boring. Now, EvaKiss isn't that good of a writer but even the worst writes know that you need conflict for the story to move forward. Jeremy and Lena being discovered would be just that. Just look how Chapter 10 ends - Ian is reunited with his Ex Girlfriend just as he was starting to move past her.
And that's not even considering the very obvious setup that is right there with Jeremy sending Ian videos; you got Louis, Allison and Emma, so considering Lena is another girl that he can fuck is naturally the next one in line. A reach would be something like expected Jeremy and Cindy to hook up because, I don't know, he saw her and Ian sleeping and is now blackmailing her for sex. That's a reach; Jeremy sending videos/images of him fucking Lena to Ian, something that he has done many times before, isn't.