The game will NEVER become NTR because Kat will always love the MC and never leave him, not even in any optional endings etc. because that cheapens their relationship as a whole.
However, this IS a netorase game. Meaning the primary focus for h-scenes is on Kat having sex with other men. Kat will have less on-screen sex with the MC as the game goes by, because narratively after a certain point their love does not need to continuously be reaffirmed. Otherwise it might as well just be a normal vanilla game, and this game is not vanilla.
Expect at least some aftercare type of scenes like handjob/blowjob with reporting etc.
In a sense in this game, Kat is becoming corrupted, moreso that she finds she likes this fetish. The reason she does is because it makes the MC happy, which in turn makes her happy.
If the MC were to ever tell her to stop, or stop liking it, she would stop.
But that won't happen, because this is a netorase game.