It's real easy. Don't push Val to be interested in other men, and don't be stupid in cheating on her. The two major NTR flavored paths are encouraging Val and going down a more sharing/NTR-lite path, and the other is straight up revenge fucking after you get caught drunk fucking the office slut at a party that you both didn't take Val to and didn't tell her you were at.
You can absolutely fuck around behind her and not get caught. Hell, you can even get her into a threesome or foursome (Evon, Alyssa, Val & MC), depending on how things playout at that very same party if you don't take Val with you. I mean, you have to pretty obviously work for it to get Val to sleep around on you.