[DM-FC (Mutsuki)] Renai Soudan... Hic... Shita dake no Nanoni... [Chinese] [Digital]

It's just like porn with stupid introduction scene. The reason is not important, what the author need is just an excuse to push the girl to have sex (rather than just show them already having sex from page 1). Most came for the situation, a girlfriend stolen while the boyfriend can do nothing, just cooked a bit differently. Oh, and the hot sex that rarely came up in vanilla.

NTR plots are all stupid, the BF only needs to take picture of the teacher doing the GF, send it to her parents and he can get revenge on both of them. But hey we all knew NTR authors had to make MCs retared in order for the plot to work.