The "NTR Route" is per-girl. You either let her arc play out so that her non-MC sex scenes happen, or not. AFAIK the only thing you can get locked out of is "Virginity lost to [MC name]" in her sex stats screen. The vast majority of the NTR content seems to be just fooling around with MC's consent, or MMF threesomes he's in. There are no scenarios I know of where you're locked out of a relationship with a given girl, just ones where certain sub-quest outcomes mean you can't tell her to stop fucking the other guy(s) once she's started, and/or cause her to start.
Affection is exactly what it says. Wanna progress anything with her? Be it sex scenes, skill-training, or a main story section she's involved in, probably need to raise it some.
Corruption is how much of a perv she is in her own right. In other words, it unlocks more positions during her sex scenes, and fewer-but-kinkier scenes than Affection. Nobody does anal with less than 10. Very little requires more than 20, and most of it's NTR.