The way Yuni is acting seems realistic to me, most cheaters never see something wrong with that they're doing because according to them it's justified depending on their own perspective.
Nanase's reaction is how a percentage of people would react when they find their partner is cheating on...
I feel like this is trying to portray how easy we are as humans, most of the time we are driven by emotions, at the end not everyone has the same mental state.
At least that's what I'm getting from this.