My girlfriend likes to pretend she is 13 when we are in bed together. Which I told her is weird and pointless, since she will be 13 next year
I'm not really sure what you intend with this comment? Are you joking about being a pedophile?
It's just stupid, I always had to keep reminding her not to say 'yes daddy' or 'I'm sorry' while she was giving me a blowjob
Pedophilia is a very serious matter that causes real harm to a lot of children. This isn't really something you should make light of with a joke.  There are some great documentaries out that can educate you on the subject.