So, the focus here is on the interactions between boys/girls or men/women.
Would a dance where just one gender participated be sinful?
To me, that's what I think of as INHERENTLY sinful, if it would be sinful regardless of extraneous circuмstances.
Then, does this criterion extend to certain types of dances that would be with mixed gender and yet are so designed as to incite very little tedency to impurity?
Of course, there are others today that are probably little more than fornication with clothes on.
I imagine that there's a gradation of sinfulness here, and at what point (with what type of dance) does it cross over into grave sin? Or, even, at what point does it cross over into even venial sin?
Another aspect might be subjective. If young teens are dancing, with their raging hormones, probably a much greater occasion of sin than if a some octogenarians were dancing, etc.
So there's a very subtle blend of subjective considerations here.
And even when dancing alone, a woman could make herself a temptation, and yet part of that might be due to her attractiveness and her grace/athleticism, where a less attractive woman who's out of shape migth cause no occasion of sin.
Apart from the aspect of sin, I for one by nature despise dancing. It's completely contrary to my temperament. I consider it undignified, in any form ... even if it were some "high-class" ballroom dance. I find it to be incompatible with holiness.
So, could you imagine either Our Lord or Our Blessed Mother dancing ... even if it were in the most modest manner possible and not in mixed company? To me it speaks to a certain frivolity, and the motions of the body seem to draw one's mind and nature down toward the carnal, the animal, and the material world. That might be just me, and this doesn't speak to sin, but I personally detest dancing. I only ever danced once in my entire life. I tried for about 2-3 minutes, found it to be so incredibly awkward and undignified (not just because I wasn't particularly good at it) ... and then immediately left the dance feeling as though I had degraded myself, and never danced again (I was about 18 in my first year of college). It's not that I'm particularly uncoordinnated, as I was a pretty strong athlete in the day with good coordination, balance, and strength. It was just the gyrations and movements that I somehow found degrading and undignified ... I don't know, somehow animalistic.