I have always supposed that one is assigned a room by the administration, but that there is an option to apply for transfers to a different room.
I also assume that some students will find themselves transfered from one room to another quite often, as the adminsitration might want to keep the targeted students away from the others.
(Sorry, Miss, a pipe has been leaking and we will have to move you to another room. Water damage, you see.. Lots of repairs. Your room mate? She has already been given a new room. Just moved actually. )
Players do, off course, decide between themselves who should be living with whom.
And sex isn't mandatory, AFAIK, it is just entertaining
