How about if I tell my new roommates that I want a poster in the bathroom taken down?
Why do you want them to remove the poster? Is this a joke or are you dead serious?
It disturbs me and it's not appropriate for the house
It's okay to want to have input on how a communal space is decorated. Try talking to your roommate first about the poster before making a demand, otherwise they may become upset. Offer a compromise where you both approve of the decorations in communal areas.
I'm just going to grab it off the wall and wake up my roommates if they don't tear it down.
They should have consulted you first to see if it was okay. There should always be an agreement like that n any household. But destroying the poster would prove to be irrational and they might throw you out.