Based on the overt reaction to getting kicked in the balls, I'd say there's no real equivalent on a woman's body, because there's nowhere you can directly impact like that which at an equal level of force, would cause enough pain to force that kind of response.

That said, some of the answers here are horrible. If a girl hits you for any reason other than immediate self-defense (as in, you at least attempted to hit first) she's just demonstrated to you that she feels entitled to use abuse to get what she wants, and you should not tolerate that. Not even a slap across the face - even if you think you've said or done something way out of line. It's even less okay to hit someone who is not allowed to hit you back than just hitting in general is.