It totally happens on it's own and is controlled by the mind. So if a guy can totally get his mind on something totally stupid while he was making our or humping it might not occur. But I don't think most guys have that much mind control. I know I don't. haha!
An erection is sometimes possible to control and sometimes not. Getting an erection is pretty easy. You just need to think about girls and it will come. But sometimes it comes when you're not expecting it and for absolutely no reason. Stopping an erection is the more difficult part. You have to think unsexy thoughts. But it's not as simple as that, the guy REALLY has to concentrate on those thoughts, which is very difficult when there's an attractive girl around.
its just there...its weird I can be in a really intense situation with a girl I'm not completely in to and I feel like I have control...but if its with a girl I'm really excited about its there before I would ever want it...kind of annoying
It's probably a lot easier to control with practice. I wouldn't know as I'm a girl, but I would say that to some degree it is kind of uncontrollable. There are things you can do to maybe make it go away faster, or hold it off for a while, but I don't think you can completely stop it from happening. Idk, I could be wrong.