As people have said you were in a comic book store, in their territory so to speak, a woman in a comic book store is an unusual sight and this guy probably didn't know how to handle it >_< I'm not defending him, this definetly shouldn't have happened, you should have stood your ground and made a scene. By succumbing to such an act you let him acheive what he wanted over you and that was power. He made YOU feel small and that's what he wanted. If this ever happens again stare at him and yell out for him to get his hand off your breast, let the world know what he's just done. Make him feels small.
I would also talk to your friend Grace about what she trains in, ask her if it's not too much trouble to teach you some basic techniques that will disable a person quickly. You would be suprised how much just bending back a single finger in the wrong direction causes someone to submit. If you are of a smaller frame and don't wish to do anything violent, I would suggest looking into a style like Aikido, I personally have practiced this style and can tell you that there is basically no effort required on the person defending themselves, it is based on using a person energy against them (Going with the flow and bending joints the way they aren't supposed to go). This choice I leave purely up to you
Take Care
