Sleep with your best friend's girlfriend after a promise

If you can endure a month of not having sex with Kou, even masturbating, I'll let you fuck me all day. Those were the words of my girlfriend's best friend Arina, who told me when she wanted to have sex with her. I couldn't resist the attraction from her so I tried to endure it and after a month, I was freed up all my hoarding with her in this sneaky relationship.