My best guess was that McConaughey was into the bondage/torture stuff and knew ahead of time that it was going to be bloody from past experience (thus the plastic on the floor). I do think a similar incident is what caused his scars on his face. And he obviously had a thing for black guys since the not-really-British black partner admitted that to Zac Efron at the newspaper.
What I don't understand is how McConaughey knew the guys at the bar were also into that stuff just by looking at them. All he had to do was drunkenly wander over to them, whisper, and take em back to the hotel.