It's something I have experienced during a session.  It's also something I experienced while mourning my the loss of my father, and that really baffled me.  Erections that happen during times when there's no obvious stimulus (not even a breeze lol) are due to wiring issues, at least that's how I have come to think about it.  Emotions are unpredictable, and so are penises.  I hope the response in this thread is helping put you at ease.
Excellent description 
@AlexBoyd. We definitely have our wires crossed / tangled. In your other thread there is a good convo starting (I think) on page 6 about unwanted arousal by things like feeling a strong emotional response / connection with someone (thanks 
@Induna). Again - wires crossed.
For me this has been raising a question. I have said often, “I did not ever find anything my father did as pleasurable. I don’t ever remember getting hard.” Since realizing the abuse likely happened prior to my first memory at age 5 or 6, I have opened the door to the fact that at younger ages I might very well have been responsive but don’t remember it.
The question this raises for me, since it seems to be fairly universal that memories of abuse (even by one’s father) can be arousing and that there were physical aspects of what happened that felt good, is this: is this just one more area that I suppressed so deeply, that my statement about never feeling pleasure and never being aroused is actually not true?
I tell people, “I’ve kind of been doing a lot of things ‘backwards’ compared to most guys.” And it’s only as I progress thru therapy that I get to the real feelings that were trapped so deeply, and then I’m able to say, “I was wrong. I have those same issues / feelings. It just took me a long time to be able to access them.”
Ugh. And the answer to this is very, very important. Because I’ve only been able to unlock freedom as I’ve unlocked my true, buried feelings about things.