I don't want to see a scary movie with my gf
What is your reason for not wanting to watch a movie with her?
I'm going to leave with someone else when my friends are not ready yet
You don't have to like the same things as your girlfriend but you should still try to spend time with them. She is going to feel hurt that you left with someone else.
I'm tired of watching scary movies with her. I want to stand my ground and not go with her.
If you do not want to spend time with your girlfriend then it sounds like you need to have a conversation with her instead of ditching her. If you just leave without saying anything then you are avoiding the problem. Let your girlfriend know how you feel.