From NetFlix:

After a serial killer brutally murders several gay men in New York’s S&M and leather districts, cop Steve Burns (Al Pacino) goes undercover on the streets, where he must learn the complex rules of the underground gay subculture if he’s to catch the psycho. Karen Allen co-stars as Burns’s girlfriend in this gritty 1980 thriller, which sparked protests from gay rights groups at the time of its release but has since developed a minor cult status.

“Lurid” is the first adjective that comes to mind. The Stonewall riots occurred in 1959. By 1980 the gay movement was strong enough to protest this film which depicts an aberrant side of gay culture. Indeed the strong gay protests are described in the Wikipedia article about the film.

Expect “grungy” scenes in this cult classic with a wonderfully ambiguous ending.