I have always assumed female friends, prostitutes, staff members, and random people who aren't kpop fans who get setup with them via staff. As a BTS fan I can safely say they definitely have sex. I think certain members have less sex than others, though I can't really tell you why I think this. I also assume massive porn consumption, masturbation, and knowledge of how horny their fanbase is takes the edge off their relatively eneven sex lives.