My girlfriend's roommate keeps infantilizing me and so I'm going to just argue with her.
Why is that roommate trying to do that do you?
I don't know, but I want to be seen as an adult and they are denying me that.
It's wrong to try and argue with her though, your girlfriend might get upset that you aren't treating her well. Instead of arguing, why not just have a talk with the three of you? She might not realize what she is doing.
That complicates relationships.
You should ask your girlfriend if she can speak with her roommate for you? You don't need to fight with anyone, even if it can be complicating.