Plenty do, but few act on it.
From a piece on Salon...
Have many biographers had affairs with their subjects?
Not that we know of. Broadwell probably isn’t the first biographer to carry on a sexual relationship with a high-profile subject at the height of his or her fame, but she is the first to be caught. Other cases are largely in the realm of rumor and unsubstantiated gossip. The best example is Doris Kearns (now Kearns Goodwin), who spent many hours interviewing Lyndon B. Johnson at his Texas ranch. The author probably didn’t help matters by admitting that LBJ liked to climb into her bed for interviews. But she insists that she never joined the former president in bed, and there is no evidence that a romance occurred.