The only thing that fingering Chika before Rin has the chance to confess to her does is cause you to miss two Rin events and cause her to talk a lot of shit to you over the course of the rest of the game. It might also lock you out of the eventual Rin romance path, we don't know yet.
The main thing that sexing Sara does (outside of the Sara events, obviously) is unlock a Sana event where she accuses you of fucking her mom. It probably will not lock you out of the eventual Sana romance path.
Neither of these choices have bearing on Sensei's larger characterization. The Molly thing didn't happen because you fell onto a villain path, it happened because Sensei is a bad person (and gets puppeted by weird entities sometimes that cause him to sexually assault people). Everyone gets that event, even if you've been doing the most moral playthrough possible, because the most moral Sensei possible is still a highschool teacher who is trying to have sex with teenage girls over whom he has a great deal of both formal and informal power.
