A 33-year-old man got busted Thursday by cops in Southgate, MI -- near Detroit -- after they got a call that this dude had fallen asleep in a theater during a screening of KS' "Love Lies Bleeding" ... with the obvious problem being that his penis was out and exposed, which was photographed.

Somebody uploaded a pic that appears to show the man in question passed out in his seat at the theater -- and it looks like this was taken after the credits rolled and lights came on.

The eyewitness claims the man masturbated and then fell asleep -- but cops didn't quite go there. All they're saying is that the guy they arrested was allegedly intoxicated -- BTW, you can see what seems to be booze in the photo -- and that he was refusing to leave.

Southgate PD also claims they found illegal substances on him. In the end, he was booked for possession of narcotics, indecent exposure, disorderly conduct and resisting arrest.

If you're unfamiliar with what this flick is all about -- it basically follows a lesbian love story between Kristen and Katy O'Brian's character ... with a noir crime plot in between. There's a lot of strong sex scenes in the first 45 minutes of the movie, but none really after that.

So, perhaps this guy got a little happy in the first half of the film -- and then started snoozing as the rest of it played .. without putting his little friend away, of course.

Kristen has been wearing some out-there outfits lately to promote the film ... but you just can't buy this type of publicity. Ya gotta figure folks will be dying to see it now ... hopefully not spanking any monkeys, though. We left porno theaters back in the 20th century, y'all.