It's perfectly common for spouses to start sneaking around and having someone else on the side even in a perfectly happy marriage and home. Let's consider the possibility that naruto doesn't have the stamina that he use to because he's always saving it for his job and he's made the village his priority instead of the wife at home. He works late nights and ends up sleeping in his office or on the couch. Hinata is probably looking for something to rekindle her marriage through someone who is exciting, physically fit, reminds her of her husband,edgy and is never in the village for her husband to expect anything.
I'm sure Hinata and Naruto will work their problems out eventually. Hopefully Hinata doesn't get caught in the meantime.