For me, the only punishment I intended to dish out with strap on was humiliation and objectification. It could get a little dicey punishing someone's anus internally. Strap on, when done correctly, should hurt just a little at first and then get to the point that it becomes physically pleasurable. I went from the perspective that for a straight man to enjoy a dick in his bum, even to the point of having a release, would be humiliating. Kind of like the blend of sweet and sour.