Fuck your best friend's lover after a promise

If you can endure a month of not having sex 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.