Funny, I've alway thought I was the only one with this same question. First, let me say that I'm physician, (so I know my anatomy), I'm gay (and I definitely enjoy my insertive anal sex), I'm a bit of a perv (or else why would I be reading this website, much less contributing to it!), and pack a standard issue, 6", slightly curved downward, happily cut pecker.
Your rectum, which Mr Friendly likes to explore, is not free to slosh around in your abdomen the way the rest of your guts can. It is a curved structure, that parallels and presses against your sacrum, the lower portion of your spine, which joins up with the pelvic bones to form a sort of "bowl". The inner, concave portion of the bowl faces forward, as does your rectum. So technically, if your buddy is bending forward, and you are taking him (or her) doggy style, the "ideal" dick is going to be a down bending one. When he bends forward for you, he is bringing his rectum more parallel to Mr Friendly; once you penetrate past his valiantly resisting anus, you enter the stretchy, somewhat fixed positioned rectum and your downwardly curved dick is going to be comfortably ensconced into his downwardly curving rectum. The bottom of your dick is curving around Mr Prostate and the base of his bladder. At this point, an upward curved dick might poke the prostate a bit more, and the badder a lot more, but you are going to be bent in the wrong direction, and depending on how big you (really) are, it could be uncomfortable, or at least less comfortable than it might be otherwise. Similarly, if you are the proud owner of an uppy curve, having your buddy on his back will facilitates your penetration. He just has to get his legs out of your way, and you are both good to go-doesn't matter too much if his butt is high in the air, his legs mounted on your shoulders, his feet on the bed, or his ankles in your hands. You're in, and you can both get down to business.
My to bits.
