for what I know, in the NTR genre, there are 4 types of usual endings:
1 - Dude get depressed after her is stolen imploding and falling in self destruction... in some case kill himself not far away from the stoling event.
2 - Dude mentally break in some way but stay with her knowing that she get more splurts than a sea rock on the coast, the hentai Otome Dori is like this if memory serve me right.
3 - They remain as couple but she keep everything hidden in a situation "I love you but I can't be satisfied by you", there is a hentai like this where highschool girl is blackmailed by fat bastard janitor because she have a relationship with one of the teachers, it end with her still be with him but despite this get other men bang her because boyfriend can't satisfy her anymore
4 - Guy get depressed after loose the girl but then recovery, after some time, and continue with his life... in a case, if I remember right, there was a story where girl is stolen, guy get depressed, another girl help boy but she is stolen too andguy basically totally collapse... not sure of this full sequence anyway, not a fan of all this dramatic development.