In that comic, they had recently started dating.
Ever since Frisk was thrown in jail.
Not wearing stripes is a rite of passage. Once the child turns 18 years old, they stop wearing stripes to show they are an adult. (Or they can be rebellious teens and forgo clothes altogether.)
Send us a link to the fanart in a submission!
This is an underground where gender norms are not a thing. Where anyone can wear any clothes because they want to.
Where anyone can be whatever they want, regardless of “traditional” roles.
Just like the canon underground.