not a chick but i can give you a pretty reasonable answer, EGO...most guys like be able to pleasure their s/o especially in the sack. For you not to be able to do so could and probably would effect your ego perhaps not simply one time but if it happened time and time again it probably would. Imagine how she would feel if for some reason she couldnt get you excited or couldnt get you off, she might feel you dont like her/find her attractive. This is the same in reverse she may want you to feel that you do it for her physically, emotionally etc...Personally Id rather her just say alright this one is for you instead of faking and perhaps if you explain that to her she might do that from now on...question is how did you not know it was faked?