There is no way I am giving my master bedroom for a guest, except in the case of parents/grandparents if need be. If I have guest over, however, I make sure that I am a good and and hospitable host, making sure the guest is comfortable, has a nice bed, and good stay.

In the case of this thread: The paying party gets the first say of who gets the bed/sofa bed. If the paying party wants the bed, that person should get the first dibs. If I am a guest of this situation, I would not assume anything otherwise than a sofa bed. Otherwise, the other party can just pay for the own bed/room and there would be no issue like this thread.