People petting without asking

I need to vent. I have an adorable 4 month old puppy who is half lab and half rat terrier. Being half terrier, she's very smart but very stubborn, so I'm training her early. Potty training has been a success after 3 weeks! That said, she still needs work on a leash. It's difficult to keep her focused when half the people I walk across don't ask to pet her and walk right up to her. At 9pm, I had a group of college boys come up behind me and crouch down to pet my puppy without asking. It startled me as I didn't know what they were doing at first, so I turned around and gave them a bad look. When I realized they were petting the dog I told them they needed to ask me first and to not come up behind a woman they don't know. I didn't know what they were trying to do at first. They got angry with me and called me a b*tch, and kept trying to call the dog to get her excited even as I was walking away from them. They followed me about a block, then gave up. I don't think my response was unreasonable. I've noticed people, particularly young men, get VERY aggressive with me when I ask them to ask me first. What have you guys done about this?