The 59-year-old man, who cannot be named for legal reasons, pleaded guilty to sexually assaulting the 31-year-old man in January 2018.

"The complainant was in a vulnerable position," Judge Vicki Loury told the Brisbane District Court on Thursday.

"He had previously been sexually abused, which you knew, and he had a significant intellectual impairment, which you were well aware of."

"He had given you no reason to think he was interested in you sexually at all and you knew he was a heterosexual man.

"Despite that, you violated him in a despicable way."

The court heard the men were friends and had been reading a religious book together when the man began rubbing his victim's back.

"He told you stop," Judge Loury said.

But the older man did not stop. Instead, he undressed the younger man and started sexually assaulting him.

"He started banging on the couch to try to tell you to stop ... He was clearly distressed," Judge Loury said.

"He trusted you and you betrayed that trust in an appalling way."

After the attack, the man admitted his actions to police saying he had been drinking heavily and believed the sex was consensual.

He was later forced to move from the apartment complex where his victim also lived and lost his job at an aged care facility.

Judge Loury said sexual assaults can have unique and long-term effects on victims.

She sentenced the man to 18 months' imprisonment suspended after four months.

Australian Associated Press