It depends entirely on the situation. As self defense, women should be allowed to hit men, and men should be allowed to hit women. It could be a life or death situation, where one hit could potentially save your life. If this is the case, then both genders should be able to hit eachother.
Women should not be allowed to hit men and men should not be allowed to hit women. The reason why domestic violence against men is much more under reported than domestic violence against women is because on average men are physically stronger than women and due to this people underestimate the potential damage a woman can actually do to a man if she abuses him physically. Just because women are generally weaker than men does not give them the right to hurt men and just because men are generally stronger than women does not give them the right to hurt women. We are talking about equality here, not female domination!