Look how marriage turned out. It was a poorly done gimmick. Look how six is implemented in the other games myk_h mentioned; it's so corny it's unbelievable.
If you want some 'alone time', there's plenty of sites out there.
This, this, and more this.
All It would be is incredibly offensive adolescent boy pandering, and it isn't like Bethesda at all.
It's not about the rating of the game, either. It's the fact that Bethesda games aren't made to be superficial and silly like Dragon Age, etc.
And if they're gonna allow six in skyrim, at least allow same-six couples to have six too.