I've had two boyfriends with this exact issue, and it's incredibly disheartening. With one, it was just that he was getting older and his testosterone was getting lower faster than the norm. He got on hormone replacement therapy and that fixed it right up. But with the other....

Well, at the risk of sounding like a drama queen, and with my sincere apologies if I am totally wrong, with my other boyfriend it turned out that he was a sex addict, and his "drug" of choice was porno. Through psychological conditioning, it turned out that he'd "trained" himself to really only finish by himself to visual-only images. Digging deeper, he had a serious issue with intimacy; he was terrified of it, so hid in porno, but longed for it at the same time, so he turned to porno as a halfway point.

So.... two ends of the spectrum, I guess. Hopefully it's just the first one or something else equally innocuous! Good luck!