Brokeback Mountain is about two cowboys who meet and fall in love in 1960s Wyoming.
"It was not the subject matter," said Diane Wiffin, of the Massachusetts Department of Correction.
"It was the graphic nature of sexually explicit scenes," she explained.
Brokeback Mountain, directed by Ang Lee and which won three Oscars last month, had been shown to inmates in the state's largest prison in Norfolk, about 40km (25 miles) south-west of Boston.