My girlfriend and I are not married yet but we decided to live together for a while. She works in an office and I work as an engineer so we both have a stable income. One day, her best friend Rin Natsuki came to my house and wanted to stay for a few days because she had just sold her house and had not found a new place to live. My wife had asked for my opinion in advance so Rin staying here was quite normal for me. However, her beauty caught my attention, that night I went into the bathroom and saw Rin taking a shower, I peeked and smelled Rin's underwear. The next day my girlfriend went on a business trip, at this time Rin had something to say, she saw what happened to me yesterday and then I was scared and embarrassed. It was a surprise when Rin turned around and asked me for sex, how could I refuse?