Guy likes Rin-senpai. Senpai doesn't want someone weaker than her, guy doesn't care (17 confessions, gotta respect that).
Senpai : "We'll pretend to date for one month, if you can make me admit defeat during this time, I'll date you for real." Guy : "Any kind of match ?" Senpai : "Yeah." => Sex battle !
Ahegao sex ensues, and you can probably figure out the rest with the pretty pictures.