I dated a girl for two years who used to get picked up on ALL the time (think of it this way... my mom saw a movie with Gina Gershon in it and said, "She looks like your girlfriend!"). Guys would start walking the opposite direction and follow her home from class, trying to get her phone number. I put that kind of on the same level as ass-grabbing... they're trying to get something they can't have. It's actually kind of amusing, as long as you trust your girlfriend.