If you want to know a real reason, you have to read up on some MaleVsFemale psychology litterature, that is written by REAL psychosists and not the "self help" kind. Aside from millenia of guys and girls basicly having different societies where they live and thus different rules to abide by, there's a lot of psychology behind the whole thing.
It's going to take a lot more than the "equality between genders" wave to get this out of our systems.
I never really read too much up on that. I was raised a gentleman, so I'm already in the "hitting girls are wrong" grouping, and I'm fine with that
There's also the upperbody strength and fair fight side to the debate, but if your really need to hit someone your either a prick or in trouble anyways.