I don't want to have sex with my girlfriend.
Have you ever had sex with her before?
Yes, we usually have sex a few times a week.
You should never feel obligated to have sex with anyone if you do not want it. Sex is a mutual expression of love and should be performed with someone you want to share it with.
I think I'm going to tell her how I feel.
I think it's great that you're going to talk with her about it. I'm sure she'll understand how you're feeling.