Limp dick during anal stimulation/ penetration

Do you men recognize this? The moment I am rectally stimulated or penetrated, my penis goes limp. A doctor explained that an erection requires active tension in the muscles around the pelvic floor. During penetration, these muscles actually need to relax to allow the penetration to proceed comfortably and smoothly. This relaxation often leads to a physical flaccidity of the penis. But I also hear from friends that they don't recognize this and are always hard. What about you?

Stimulation…

Not always, but yes it happens, probably more often than not. I put it down to focusing on other sensations 😁 but the physiological reason sounds more likely.