Astolfo is a femboy icon, and while there's nothing wrong with being a femboy, the community does skew younger. At a certain age, it gets awkward to call yourself a boy or girl, as our bodies naturally mature. It has a lot to do with the loss of youth, and the shame that comes with it.

Teachers are expected to be more "grown-up or mature," with them using cute characters coming across as out of touch. It gets awkward when teachers break that perception, like they're trying to be someone they can't.

If the teacher doesn't know who Astolfo is, they seem even more out of touch than if it's a cute girl character; a "how do you do fellow kids?" moment. If the teacher is aware and intentionally represents themselves as a femboy, it still breaks the view students have of teachers as mature. It would be abnormal instead of immoral, but that alone is risky if a conservative catches wind of it.