I don't have much time and may not have any to reply as I am going away soon. I used to masturbate more than I do now. Opportunity lessened and some desire waned so I virtually stopped. I think I lost it because I wasn't using it enough. I have erectile disjunction and need Viagra or its equivalent to have intercourse but can still masturbate. That's more to do with the fact that I don't need to stay as hard to masturbate as I do to have sex. My penis can go a bit soft and I can keep masturbating but any loss of hardness to that extent during intercourse will stop things completely. So I don't think it has anything to do with relative turn-on stimulation or feelings during intercourse verses masturbation. It's purely a mechanical thing and his distress and worry at non-performance is very typical. His desire and need to ejaculate has not lessened but intercourse becomes a challenge to perform, while masturbation is much as it always was.