I think that the books and films make it quite clear that Hogwarts has boys dormitories and girls dormitories. Hermione goes to Harry and Ron's dorm once or twice but she just visits and doesn't stay there long. So I think it's quite clear that Hogwarts only has people of the same sex in the same dorm. So why do I read fanfics and see people of the opposite gender sharing the same dorm? It honestly boggles my mind why Dumbledore would think "Hmmm I think Miss So-and-so should share a room with Mr Blah-Dee-Blah. No mature things will happen and Miss So-and-So will most definitely not get pregnant!" No. No. No. Dumbledore would never let people of the opposite sex share a room please stop writing your stories like that. Please.