Maybe they didn't go there for sex. Maybe they just needed a safe place to shoot up.
This isn't just a Japanese phenomenon. In the past few years, I've seen a lot of articles and news stories about hooking up. But part of me thinks this is just the talk of old people. I mean, when I was about 12 I knew of four girls in my school who had to take time off for their pregnancies. It happens, but it doesn't mean everyone is doing it.