Would it be weird if I told you that I've masturbated thinking about my girlfriend's daughter?
Are you joking? Why would masturbate thinking about her daughter?
I just like thinking about her as a sexual object, that's all.
It is not good to masturbate or have sexual thoughts about children and if they are not a child it is not good anyways. Your girlfriend would be upset and sad if she knew. I'm sure your relationship would be over.
Honestly, I don't care. It makes me feel good.
You really should care. If she's a child that means you're having pedophilic thoughts and you need to get help, and if not, well you're just being a terrible person to your girlfriend. I really hope you can think about what you're doing and understand why it's so wrong.