OK so this is embarassing for me to ask but I don't know who else to get advice from... So basically I've been playing with my 'toys' and I've stumbled across something that worries me. Basically when I'm got a toy 'up there' it feels really good, sometimes get a bit of a quivering sensation which feels great...unfortunately either I'm doing something wrong or my damn arms are too short because I can't actually reach far enough down to keep the motion going long enough to really enjoy it. However that is not the problem. The problem is while I'm at it I've noticed that my penis stays totally limp! Now it doesn't help that between trying to balance and move a toy I'm left without much in the way of arms to stimulate the poor bugger but still, I was kind of under the impression that when I'm enjoying something sexual I didn't really NEED to... Is that supposed to happen or is something wrong here?
Save for the "wave", I experienced the same thing as you in the way of a flaccid penis. I think it's normal. Lots of guys in porn go flaccid too, I noticed.
There toys specially designed for prostate stimulation that are simply inserted and do their thing without any other effort on your part (some vibrate, some don't). Maybe you could look one of those up. Your penis might be going limp due to being nervous, or afraid it will hurt etc. Try to make sure you have total privacy when you do this, and that it's unlikely anyone will bother you. You'll be able to relax more then.
It's perfectly normal to be enjoying anal pleasure and not have an erection. There are some other threads where we have talked about this but anal pleasure doesn't always result in an erection. The way that porn is edited, it gives the impression that guys who are bottom have hard-ons for the entire time. That's not the case- whether you are alone or with another person. On the other hand, if you're using a toy for anal pleasure during masturbation, it sometimes makes more sense to get things started up front and then use the toy to add a little something extra to the experience.
If you are concentrating on stimulating the prostate and not the penis your body will automatically send blood to the prostate, just like getting a hard-on. It's like the trick of flexing your muscles to get rid of an unwanted boner because your body will concentrate on sending blood to those muscles.