People catch a whiff of NTR and lose their minds. Hold up, ya'll, it's not gonna happen. I'm calling it right now- new sex positive character will act to get our main couple to finally bone. She'll either teach them about how to have safe sex and teach them not to be shrinking violets about it. OR she'll make them jealous so they make a move to protect each other. It's an old trope, but it checks out.
BTW, why are we mad about NTR that hasn't happened when we could be mad about black boxes that did happen?