You have a childhood friend by the name of Tsumugi Tamura.
You loved her and you thought your feelings for her were requited...
But she turned you down.

You've graduated and now you have a new life, and a beautiful, wonderful, girlfriend.
What more could you ask for? ... Well, perhaps a little sex would be nice.
That's right. Your girlfriend is everything you dreamed for OUTSIDE the bedroom.

And so today, like everyday, you head over to Tsumugi's place.
"You can have real sex with me. The kind that you don't get to have with your GF---" she says.

Whoa. No wonder you cannot stop having sex with her, EVEN THOUGH you have a girlfriend!

(Source: DLsite, official)