This story has some NTR elements but is somewhat limited in scope.
Yanagi gets to date Tachibana for a while, and is the only other man capable of touching her (including teaching her how to swim at the pool). She develops some feelings for Yanagi but still feels extremely strongly for Kirishima. They never kissed/got intimate.
What?? Isn't hayasaka the one who likes that man? why tachibana somehow get involved there despite having fiance and MC? could you please provide some explanation? Thanks @uHikari