Since the total personality disintegration that followed the loss of his drums, the Master was passive. Compliant. But he still had needs and sometimes, when the Doctor sat down on the couch, the Master curled tightly against his side and pressed his erection into his hip.
He should stand up and walk away, the Doctor knew. The Master wasn't capable of informed consent. But when the Master tucked his head against his shoulder and began to rock, the Doctor slipped an arm around his shoulders and pulled him close. He held him like that until he shuddered and went limp.