It's cool, as long as the two of you have a signal that it's no longer pleasure and has turned into pain. Most guys would probably disagree, but I don't get any pleasure out of hurting the woman. On occasion, some husbands ask me to "pound her deeper," or "fuck her ass," but if I sense it's uncomfortable for her, I'm not going to continue just to please him (especially if I sense that he wants to see her abused and/or hurt). Be careful because it's a thin line between her wanting roughness and/or pain for pleasure and wanting you to do it to make her into something cheap and undeserving of respect.
