The blackmail is the starting point but if you make choices that a caring and loving boyfriend would, that's all there is to it: Blackmail.So for you he deserve to be cheated on because he trust Liz, believe what she say, respect her wishes...
The blackmail is the starting point but the problem here is Liz attitude... In the ntr path she avoid MC, she cancel almost every date with MC because of work or being too tired, she lie to him when she say she is going home and instead go to Matt place...
She will cheat though of you act like an asshole and neglects her all the time.