Why has my Dad suddenly become obsessed about sex?

My dad (will be 81 this December) is in an Alzheimer's/Dementia section of a local nursing home. He has suddenly started to think that 1) the medical section on the 2nd floor is a brothel, 2) the staff insist he must select one of the women residents as a sexual partner, 3) because he does not want to do that, he thinks his nurse wants to marry him. He has had prostate surgery and is probably impotent and has been widowed since Feb., 2001. Is this something common to Alzheimer's / Dementia patients? Sorry if this is a troublesome topic, but we are looking for answers.