The impression I got was that subsequent encounters were physically traumatic for her, but that was due to issues of size, frequency, being unaccustomed to such activity, and all of that being piled onto the aches and sores of riding all day, every day with equally little experience. All Drogo is guility of from my reading was being unaware of her cultural sensibilities. The only two times we actually see the event in question he is amenable to her demurral.