Well everyone can finally put down their cheating pitchforks now lol.
It seems to me that Fukunaga has something like a cuckold fetish? Overall looks like a healthy poly relationship to me and the reason why Tsuzuki gave off bad vibes was because she's already on edge from the train encounter and tired of being judged for her way of life.
Really loving this series so far and eagerly looking forward to seeing how the two mcs affect each other's lives.