I'm of the belief that if they hit you first, then fine, they started it and you are defending yourself. But know when to let up (this goes with anyone) and don't beat the s**t outta someone for the hell of it. That being said, girls...realize that guys can and usually do hit a helluva lot harder than you do, so be smart and don't get into a physical fight with them unless you are ready to get beaten up.
Edit:
However, if you hit them first, yer an a** and deserve whatever comes to you (this goes for anyone). Also, if you hit them because of words said...yer a ********. Just sayin'. And gents, realize you will probably do a hell of a lot more damage to a girl than a guy, so...keep that in mind. We have different body types and girls just aren't built to fight like a guy (usually).