Just as an alternative opinion, Miru isn't interested in an open relationship - Willi has to want it, and she agrees despite being ridiculously ready to settle down and have babies with him and only him. You have to actively keep suggesting sharing and open relationships for her to accept, and she taunts him about Katie, probably as a slight revenge for making her think of sharing.
Miru is also clear that she loves Willi and doesn't want to share the feelings, that if they have something open, it can't be romantic. She doesn't want competition for his feelings or attention in any other part of their relationship.
As a set up for NTR though, it works great - Miru isn't keen, doesn't like the idea and will tease Willi about it, trying to make him awkward about his attraction to Katie. If Willi ends up going for Daphne, he's definitely cheating under their agreement, and Miru will extract revenge in some very NTR way I'm sure.
I'm not into the NTR, but I do like the idea that Willi can't mess around without consequences like 90% of AVNs.