Well, I think it can also be interpreted differently. Secret relationship. Secret from whom?
If you think of it as Mario, Luigi and Peach facing the kingdom, and having to hide that they're a vee, the way they're standing and the way Luigi and Peach are holding hands make sense. Hidden, not from Mario, but with Mario's help, from everyone else's judging eyes.
I think that's something a lot of poly people can relate with.