My wife's friend's obscene muscle training act has driven me crazy...

I've been feeling bored since I haven't been exercising recently, and at my wife's recommendation I was asked by my wife's friend Yui, who used to work at the gym, to teach me how to train. Yui continues to train on her own after getting married, but she has a very beautiful body and was nervous just because she was training alone. On the day, when Yui, who was wearing sportswear that fits her body, taught me how to do step-step muscle training, she unconsciously got an erection...