Gemini, I don't think it's a matter of manner or of fear, its a matter of male pride.
First off, how did you let a girl hit you? Maybe a sucker punch once but after that? Male pride dictates that I will not admit that someone else was faster or smarter than me.
Secondly, how much does it really hurt? Male pride dictates that even though we failed to avoid being hit we must not admit that the punch hurt us. Smiling and showing no effect from the punch solves this primal dominance problem. Showing pain or admitting that you were hurt by retaliating is a failure for male pride.
A man never hits a woman. Not because he is too kind but because he is too proud to be admit he would need to.
I guess I either don't have enough male pride, or I am too much of a feminist or something

I'm totally prepared to admit that a woman is capable of physically hurting me. I'm also enough of a vindictive b***ard that I would want to payback with interest someone who hit me, regardless of their gender.
Probably just my own perspective, but I really think it's important not to let people take advantage of you. If you let yourself get smacked by a chick but let it slide because of 'male pride', she is going to think she can do that to you again with impunity.
And anyone else that witnesses it is going to form some kind of opinion of you based on that as well - although of course, that may be a good thing! "What a decent guy, she socked him in the face and he just walked away". But for me personally, I'd rather not display weakness and instead assert my desire for equality ;> Act like a b***h get smacked like a b***h.