It is up to the Top/Dom. If that is what he enjoys, he can slap the shit out of me. I'll offer my face like I offer my pussy. He is to be thanked as well. Speaking on a personal level, I do happen to enjoy and encourage it, slapping me in the face. It makes me very excited during sex. I have been used by guys that start off as gentlemen, and as they are using my mouth, when i grab their wrist and bring their hand to my face in a slapping motion or I slap myself in the face looking up at them, they start slapping me and become more comfortable acting like themselves. The alpha in them comes out and I can feel them using my face to relieve tension. The harder slaps hurt. It is afterwards when I am reflecting on it that all that pain and hurt has suddenly converted into satisfaction and high states of arousal.