I'm on your side @Rihana . I would not enter my guests room (except to clean it with prior intimation).
I fully concur with your views.When my second daughter stays in the guest room with us I dont enter her room in her absence.That amounts to prowling ie atleast what the guest may have a lurking thought.
We are hosting a young adult around 21 yrs old a few times this semester, he leaves wet towel on the bed, sometimes moves dirty suitcase to carpeted area : ) forgets to open window as directed, and throws banana, orange peel into trash can in room ... drives my DH nuts. I will seek solace in a lunatic asylum if this happens in my house.I am not surprised at the reaction of your DH.Under such circumstances he has every right to enter the room and open the windows.In my case when this happens with my niece I not only enter the room but ensure that every thing is cleared by her under my supervision. In fact I have attained notoriety in family circles to such an extent they go to great lengths to ensure cleanliness.
I would side with your DH. Well one should know what is the guest upto. However, I wouldn't enter the room, if the guest is there. Guest also shouldn't expect hotel like privacy.
I also agree that you should not enter the room when the guest is there. However you can inspect the room when the guest has gone out and tidy the room when the maid is cleaning the room like they do housekeeping in the hotel.