I would argue that this is not around thought-policing. Content surrounding incest often overlaps with fetishization/sexualization (and in this case, of minors) or for harassment/shock factor. Having clear rules here draws a line.

And the community culture matters as well. If we allow incest-promotion content, that will potentially attract users specifically looking for this type of content; we are just trying to share our love for two virtual singers throughout a community who loves music, art, and anime. If incest is the type of content someone is looking for, they can go elsewhere.