Why "lets stack together" is nsfw?

To stack things means to put them on top of each other vertically, like a stack of plates.

When one person is stacked on top of another person, their genital areas are normally near each other, so it takes on a sexual connotation. Or if one head is toward the other person's feet, they're in a "69" position, which is also sexual.

Piggybacking is not stacking, because it's not a vertical arrangement. Also, stacking usually means that you can have numerous items in this arrangement. But except for acrobatic troops, I think people would have a hard time piggybacking on a piggybacker.