Seems like right now up until recently the LW could pretend their kid isn't really trans and is just quirky, while humoring them by using different pronouns. This is especially helped by the fact that they came out during the pandemic and so haven't really been outside the house much. But when their kid uses the "wrong" bathroom they're confronted with the fact that this is actually real, and they're probably worried that it means their kid might even want to transition and be the gender of the restroom they're using, which they definitely don't seem like they would be okay with.