Cruising (1980) on Netflix

Severed male body parts have been found around New York City. These discoveries happen concurrently with a few gay men, they being involved in the subculture of leather S&M, being murdered, their arms and legs bound behind their naked bodies, and multiple three-inch deep knife wounds in their backs. The police believe the two sets of incidents are related. Lead investigator Captain Edelson offers the job of going undercover into the leather S&M gay subculture to find the murderer to police constable Steve Burns, who matches the physical type of the victims. Despite being straight, he accepts the job as he aspires to be a detective, but he is told he can't tell anyone, even his girlfriend Nancy, of his undercover work. Steve goes through a steep learning curve about this gay subculture as he tracks suspects. As he goes through this undercover work, he has to decide how far he will go to achieve success. But he also is highly affected not only by what he sees in this subculture, but by the police's reaction to events that are happening around him. This may affect his ability not only to carry out the job but in his personal relationship to Nancy.