This is about Japanese values, I think. They show the people on the "orgy bus" as completely unsympathetic, not just for their wanton sexual habits, but for only thinking of themselves and letting a madman have control if it means they can keep acting out. This is why the ultra-national group (the people who own and control the mansion) are considered sympathetic: They may be authoritarian and demanding, but at least they are genuinely acting for the benefit of the people they are helping and the entire area would be overrun if they weren't in control.
It's also probably so the group dynamics wouldn't have to change. Right now it's a team that works together, but the second there is an official couple, the dynamics change and you can't always count on those two in every situation.
Regardless, it really doesn't matter in the long run: Unless the author/artist comes back to the series (it's been over a year since he did an issue and he stopped right after they found another kid's mother), there won't be a second season.