Kanojo ga Sawararete kara...

The protagonist loved her with all his heart. Having to fend off other men all the time made him a nervous wreck.

One time, when his girlfriend was crazy drunk, the protagonist saw her being groped by a lewd stranger. Seeing this was actually very arousing...

From that day forward, his own lewd satisfaction for seeing her in such situations overwhelmed him...

[From DLsite English]