'CSI' -- Straight Crew Member Sues Over Alleged Gay Bashing

A gay-bashing supervisor at "CSI" tormented a crew member with homophobic slurs and crude drawings of penises ... because he THOUGHT the worker was a homosexual ... according to a new lawsuit.

Donald Perdue -- a set painter on the show -- claims his supervisor, Jeremiah Redclay, engaged in a 5-year campaign of brutal harassment ... calling him "fa***t" and "c***sucker" and hanging "sexually disgusting" drawings of gay sex at Perdue's workstation.

In his lawsuit, Perdue -- who claims he's straight -- says the harassment started when he heard Redclay refer to certain actors on the show as "d***suckers" ... and he asked Redclay to stop.

And this is weird ... Perdue claims Redclay lunged toward his genitals on one occasion and when Perdue pushed him away he claims Redclay responded, "Come on Mucho, you know you want it."

Perdue claims Redclay retaliated by making him a laughingstock at work ... which he endured for 5 years before leaving the show on a medical leave.

Perdue says he's the victim of discrimination ... simply because his boss THOUGHT he was gay.