In discussing this, Rav Ashi said to Ravina, "What point is in question to you?" You should be able to resolve the inquiry on your own, as follows. In a state of erection, you cannot find such a case. Therefore, you can only find such a case when the organ is limp. According to the view which says that one is not liable when he copulates with a woman forbidden to him by using a limp organ - here, too, he is not liable. On the view that one IS liable for such copulation in the case of a forbidden woman, here, too, he is obviously liable to two sin offerings: one for committing the copulative act and one for submitting to the copulative act.
Art: Young Man in Reverie by John Singer Sargent