I agree with what just about everyone else has commented on. I have one mare, Love, who is seriously like a puppy dog. She has great ground manners, but when you mess with another horse, she is like white on rice on you. She wants her lovin too! I have a lunge whip that I can smack her butt with, and I also use the rope halters with the leather poppers on the end. I will use that in a heartbeat. Your safety comes first. If another horse is threatening your space/safety or your horses, I say you defend yourself. I don't hit in the face, rather the chest, butt, side...whatever, but NO face. They all learn pretty quickly. If someone gets offended, then explain why you chose to do what you did.
Yesterday is history, tomorrow is a mystery, and today is a gift-that's why it's called the present.