I mean, if I did this, I would still have absolutely no elaboration on WHY people have the opinions they do, which seems to be your stumbling block.

Really, Maia's motivations are right there in the text: she wants to indulge her little sister, and it feels like a challenge. Vikki is being a little kid thinking it's awesome and refusing to back down, there isn't much to it.

Your real question is "why are they so casual about it that these motivations are enough" but the answer to that is "because it's a guro porn story".