Am I being unfair about my husbands daughter?
We're about to move into a bigger house with a plan to start our own family. My husband has a daughter from a previous relationship who is almost 19, she lives with her mum and stays with us one night a week. The new house has two large bedrooms and a third small bedroom just big enough for a single bed. I mentioned in a recent discussion about my step-daughter having the smaller bedroom if we did have a baby and my husband got a little annoyed about this. He says it doesn't matter how often his daughter stays with us but that the larger room will be her bedroom for as long as she needs it. I do agree if she was younger or lived with us on a more regular basis but she doesn't use her current room anyway, she only goes in it to go to bed and she doesn't keep any clothes here anymore either. I just didn't think it seemed fair if we do have a child soon (who will obviously live with us full time) to put them in a room too small for anything other than a bed, when there is a bigger one that will be empty almost all of the time and my stepdaughter will be almost 20 by then.
I might be wrong and my husband says I don't understand as I don't have children but that he needs to provide his daughter with a stable home and not 'kick' her out of her new room when we have a child.
I would appreciate other peoples opinions on this as I don't want to be unfair to my husband about something I may not understand.
Thank you!