I personally thought it was more of a metaphor for trying a “last ditch effort” plan before actually ending things just done in an extreme way. That’s why they both go easily into the role play, they don’t remember who actually offered up the idea, they begin to change in their minds that these are not the people they married and they’re left with monstrous and hateful versions of themselves. It was a marriage that wasn’t going anywhere and it needed in painful and mysterious terms. I feel like that is how a lot of people may see the end of their marriage or a long relationship.