"Concrete" proof--how about common sense? If you bring home a bunch of barflies you meet during the course of bar-crawling--inviting same barflies for a drug/booze/sex party--what are the chances that among them will be some "undesirables" or at the very least, a lurking carrier of a sexually transmitted disease (if not lice, fleas, and ticks). People who inhabit the "underbelly of society" are more likely to harbor perverts than, let's say, at the church social. True, there might be a perv at the church social, but chances are far, far less.
I'm sure lots of us would be amazed at what we could drag in from some sleazy bar at 2 a.m.--mostly because most of us wouldn't dream of trying such a stupid stunt. Probably the best thing about this is, none of the trash they brought home happened to be axe murderers, at least not for that evening.
Could you kinfly show me where you gathered this information from?