Go nuts, the porn run IS the fun run. Almost every minigame taps into Rayne's horniness level, and that actually intended. The only drawback with hornimaxxing is that it severely weakens you in dungeon runs, so you want to take that demerit into account if possible.
This, it basically similar to Shimobashira's Enishia rules wherein everything's fine except vaginal penetration. If anything, you're actually encouraged to go slightly lewd because the game actually guarantees a bad ending if your rationality is all the way at 100% with Desire being 0%, which makes sense, since Rayne overcoming her repressed emotions IS the win condition for this game.
It's actually pretty jarring playing this back to back with Shimobashira's Binding of Enishia because in terms of design, the gimmicks they're going for are almost the same, except in Enishia you're on a timer, while Nightmare Knight wants you to go all in on lewdness.