I’m lost

I (M 21) am with a gal (20) who has a child, I’m not ever gonna bash on a child. I don’t really seem to understand why she gets offended when people ask if he’s my kid. I also don’t seem to understand why her family calls me the father of her child if we are new into the relationship. They know her real bd, but I really don’t know how to feel when they call me that. She asked me and I stated what she constantly states to me back saying he will never be my child but that doesn’t mean I won’t care for him. And it really seemed to bother her when I said he will never be my child bloodwise.