AITA for not wanting my GF’s kid sleeping in the bed with us?

So I (32m) have been dating this girl (26f) for about 6 months. Things are going well and we are about to move in together. Problem is, she and her 9 year old son are extremely codependent sleepers. I understand that a lot of single mothers sleep in the same bed as their children for companionship/comfort and whatnot, but I think it is just flat-out inappropriate for him to be in bed with us while I am there. She disagrees, she thinks that since I’m supposed to be his father figure it’s okay and I should encourage this behavior. It just makes me very uncomfortable having a child in the bed with us. Am I the asshole here?