You do know, that there actually are people that prefer certain things? Some people like a HJ more then a BJ, so yes there is a world, in which a person would want a HJ, not a BJ. So i would argue against it, that if a Prisoner forces you into a Sex position, that you can change the position, since he should be the one that dictates what happens, not you. Now if we are talking about you being the one that starts it, I could see a system, that let's you switch positions, if you have the necessary Sex Skills for it.