I don't believe Mr. Darcy meant to be condescending although Lizzy may have taken it as such. Dancing was mostly the domain of women and the lesser interest of males for dancing was greatly lamented in contemporary writings. I think Mr. Darcy was just matter of factly stating that ladies are energetic on the subject of dancing. It could probably be understood that a gentleman would be equally energetic for the subject of hunting, for example. I believe Elizabeth may see it as condescending because she assumes Mr. Darcy has a "satirical eye" (censorious), she believes he is looking at her critically and she does not want to be seen as silly and frivolous like her younger sisters and even her mother. Of course, this is all rather silly because we and the narrator already know that he is beginning to not look at her appreciatively.