I think that when you get turned on by the thought of another man fucking your wife it takes over completely and that becomes all that will get you aroused.
I used to get so hard that it felt my dick was going to split. Now, i can only get hard thinking about my wife with another man. If i stray from that thought it goes soft. So, when we do have sext together, se ends up stroking me and talking dirty to keep me hard but that means I cum in her within 30 seconds to 2 minutes.
I think this must be a common problem for us cucks.