Crusing

Cruising (1980) Al Pacino

A serial killer brutally slays and dismembers several gay men in New
York's S&M and leather districts. The young police officer Steve Burns
is sent undercover onto the streets as decoy for the murderer. Working
almost completely isolated from his department, he has to learn and
practice the complex rules and signals of this little society. While
barely seeing his girlfriend Nancy anymore, the work starts changing
him.