I wonder what's the reasoning behind the affection penalty with Rose if we let Ian bang her. Is it because she feels rejected? Or maybe she finds Ian repulsive and she's angry at Edwin for (indirectly) forcing her to have sex with the pretty boy?
In Rosies introduction scene she tells Edwin, that she was introduced to Killian by Kathleen a month ago, so Killian was her "Handler" for that period. She also reveals that she was afraid of him "just stringing" her along and the Deal with the club was just a trick. Ian sent Rosie to meet Edwin and she was told to "convince" him of her qualifications, effectively pimping her out. The fact that we loose an affection point with her when we admit to Ian being our friend, shows us that Ian, at the very least, failed to establish a good rapport with her, and more likely - given Killians attitute towards women he sees beneath himself - has already earned her aversion.
Handing Rosie over for the shoot is essentially the same dynamic in reverse, and shows her that you are apples from the same rotten tree.
Depending on your Playthrough there may of course be also other reasons, like feelings of betrayal, feelings of rejection, feelings of not taking your deal seriously and the general preference to do a highly unpleasant act rather with you than with someone else.