I think you are a little boy with a dream of having that happen to you.
Well maybe it your fantasy, but I'm still going to answer your question about male involuntary erections. I'm married now and have a very exciting sexual life with my husband. Once we went hiking in the mountains and he let me tie him to a tree. I din't tell him I was going to strip him naked and just in the time that took me to drop his pants and underware, he was hard as a rock. Just the feeling of being stripped and helpless and posibly caught by strangers almost made him cum without even touching him. But this was fore play between me and my husband. When I was a school teenager, we put quite a lot of boys threw embarrassing pranks. Three of them were publicly stripped naked from the waist down and all of them had full erections when their clothes came off. So yes it's true, a male could even be forcefully raped and still get a hard on. It can be in a willing playful game or against his will, but either way his dick is going to get hard. No matter what the situation is, he not going to be able to stop his dick from getting hard.
As hard as a rock?
I was stripped by a couple of guys down to my tighty whities in front of a bunch of girls. After the girls stopped laughing and it seemed that they were laughing forever. I had no hard on. The girls asked the two boys to take my underpants off. So the two boys wouldn't look like fags taking my underpants off, they forced me to take them off or they would beat me up. I took my underpants off slowly and when I got them off they started laughing because my dick was real small. I still had no erection.
When I got my clothes back on I went home all embarrassed.
When I got home I was laying on my bed and I was thinking about what happened to me that night. I got an erection while I was thinking about these girls seeing me in my tighty whities and then seeing me naked. I put my hand around my dick and I started to stroke it and I shot my load in less than 2 minutes.
You thought wrong.
I know what you mean.