だいすきなぼくだけのきいちゃん

The protagonist Madoka is living a plain yet peaceful school life with her lover Kiichi. 

But one night, Madoka is r*ped by Kiichi's childhood friend, Chihiro. 
On the next day, Kiichi starts to act weird... 
Although worried, Madoka worries about Kiichi. Then one day she sees... 

An "NTR" (cucking) themed BL novel game. 
The protagonist's lover who is a "top" is cucked as a "bottom". 
Please be warned if you are not comfortable with NTR, cheating, and love triangles. 

DLsite English