I’ve always thought it was sinful when it either imitates sex, or is being used for seduction (in which even wholesome dances can be used unfortunately) As an example of the former, I especially felt like this in HS (a catholic HS by the way) where we would dance with girls and basically imitate having sex and the bad thing was that a lot of guys and girls felt dirty afterwards, even if they said they didn’t.
The latter is a little harder to see, but think of it this way. Even if you are doing a dance that seems chaste (like country swing dancing, which a lot of religious groups at my school promoted and had barn dances) you can use it for seduction or to manipulate guys or girls into doing immoral things, but in that case it’s not the dance that is immoral, but the motives.