Fuck your best friend's lover after a promise

“If you can endure a month without doing that with Kou, including masturbating, I will let you make love to me all day.” Those are the words Arina - my girlfriend's best friend, told me when she wanted to have sex with her. I couldn't resist her attraction so I tried to endure it and after a month, I was able to release all my reserves with her in this secret relationship...