Time to man up sir. You need to just focus on the facts and read it for what it is.
Girls just dont go to the movies with other guys while in relationships. She has lost all respect for you and the relationship and its probably to late to fix it and if you talk to her about it, it will probably just push her away even more.
There is a few ways to handle this.
1. Be alpha about the situation. Dont get mad, jealous or upset and just move on while staying friends. I bet your a catch and could find girls at anytime.
2. Talk to her about it and try to set your boundries since they haven't already been set. If she is receptive to it and agrees, great! If she puts up resistance then you know she is set on her ways and things aren't going to change. Move on at that point.
3. Or you can not say anything, like most guys, let her continue to disrespect you and eventually cheat or leave you, if she hasn't already.
IMO I would go with the first option, you should have stated your boundries from the start. Respect is a huge factor in relationships and she has none for you. You deserve respect and she will see you in a new light once she realizes your not gonna let her walk all over you.
I also want to add, at no point should you be a dick about ANY of this. Just move on and focus on building a healthier relationship.