I would fucking get it if they're in an open relationship but he's seriously cheating. There's no justification to cheating. And to think that the uke is irrevocably in love with him. My heart breaks for the uke, darling you deserve better than this.
I mean, Taiki is a fucking dick for doing that to our innocent Yuki, and I believe he doesn't really understand the weight of doing that to Yuki but I believe they'll soon find out about that. I just hope Yuki gets all the happiness he deserves!