In general I think the "men should never hit women" thing is a vestige of the Victorian ideal of benevolent patriarchy. That kind of condescension is outdated nowadays, but what tp says in the OP is true, i.e., men on average are bigger and stronger than women and a fight between a random man and a random woman is likely to be unfair. Beating up a weaker man is equally shameful in my view.
That said, by all means, men, defend yourselves, but of course don't hit anyone unless it's necessary. Physical abuse of men by women can and does happen.
