I masturbate, but I'm going to tell my girlfriend a white lie and say I don't.
Is there a reason you would lie to her about masturbation?
Because she is such a prude when it comes to masturbation, and she treats me like I'm in kindergarten. I hate it.
Masturbation is normal but you shouldn't lie to  your girlfriend about it. It will be more upsetting to her to find out you do it anyways and lie to her about it.
I don't care. I hate the way she treats me and if she finds out then I want her to feel bad.
There is nothing wrong with masturbating whatsoever, but purposely lying to your partner is not the way to go. You don't have to disclose anything you're uncomfortable with but you should be open with her and if this a deal breaker for her then it might be time to end the relationship.