It sounds to me like you have handled the situation quite well. They are now aware that this is going to be unacceptable to you. I'm sure your son knew he was pushing what you'd be okay with, but it is quite possible that his girlfriend thought they were okay since they were both clothed. I would let your son know that you still feel very uncomfortable about this, and that he needs to be much more careful, especially in your home, and then I would recommend you give him the benefit of the doubt that as an adult he will reconsider his choice and make the right decision. If he does not and this continues to be an issue, you may need to not allow him to have her in his room.