Well she might (is) just be a bad communicator. Communication is the downfall of most modern relationships and marriages i bet you can ask pretty much every Couple Councelor out there and he/she will probably tell you Communication is key and that its absolutely crazy that most adults are not very good at it

so this is actually a pretty belivable reason to initiate the NTR story. Youll also realize / already have realized ( if you are/ where in a Relationship IRL) that it is not that easy to tell your partner things outright like "im sexually frustrated" because of backlash or reactions you fear, this especially feels to be the case in the context of a Zombie Apocalypse where Survival has Priority and you dont want to tip off /stress your partner that is actively going out into the danger Zone as a Runner and is stressed for his life every day anyways.