Mmmm I don't know that this is true. I sometimes masturbate as a stress reliever. For example when I was WFH between meetings I would often have a quick masturbation break as a kind of circuit breaker for my mind. I think however there's a difference between the effect of stress on masturbation and the effect of stress on partnered sex. I certainly know if I was stressed with my partner, especially stressed ABOUT something to do with my sexual "performance" it would ruin my ability to focus on the pleasure.