I haven't been exercising recently and my body has started to get flabby, so at my wife's suggestion, I decided to get some training from her friend Yui, who used to work at a gym. Yui, who continues to train on her own even after getting married, has a beautiful body, and I was secretly nervous just training with her. On the day, Yui, clad in tight-fitting sportswear, was showing me step-by-step how to train my muscles, and I unconsciously got an erection...