this is not something that can be done on a moment's notice.
i know a man who used to be able to do this. He told me that it took months of stretching his back little by little to be able to get in a bent-over-double position. A wall to press against would also be helpful.
I wonder how people would or should think of you? As a gay or perfectly straight person? I mean, it would be your own dick and doing it simply for your pleasure. Real weird I think...