I've experienced that phenomenon on quite a number of occasions. I assumed it was accompanied by some type of stimulation and didn't just go off on its on. That would be embarrassing.
I never get fully hard anymore without pharmaceutical assistance, but my wife prefers it that way. I've become quite accustomed to it. Now I'm not completely soft, I mean. Don't think you can get juice out of this rope.
Now with the little blue pill things change drastically. It's almost just the opposite. It's difficult to ejaculate at all, which soon turns pleasure to something different for her. I almost hear her say "enough already," but she's too kind for that.
I'm not looking for a happy medium between the two. I like it the way it is.