Of course not! In fact, you should tell your girlfriend that you do that. She’ll most likely be flattered that you picture her when there’s so many other people you could think about. It’s huge turn on and a nice self esteem boost to know that your bf/gf is fantasizing about you when they touch themselves. Why on earth would you think that it’s weird? It’s GREAT!
Everyone masturbates – even when you’re in a committed sexual relationship. In that situation you would just hope that you have less time to masturbate because you’re actually having sex…lol.