that I don't have the right to refuse a dance with you? And that you would rather hear some off the cuff reason, which is more than likely a white lie?
(Yes, I'm playing Devil's advocate.)
Though if I'm saying that I'm sitting this one out and then get up and dance, that's rude. At times, I've been known to need a brief breather at the beginning of the song and then finish it up with someone who's asked (especially after a brisk V. Waltz or Swing). Would you rather wait for me to catch my breath, and miss part or all of the song, or be given the opporunity to find another partner and dance the whole song? And does that person constantly refuse you, and why would that be?
But the 3 rudest things I have seen and heard, is the person who sneezes into their hands, wipes it off on their pants, and proceeds to dance with a partner. Secondly, as a straight, tall woman, who will step in as a leader when classes are unbalanced, I once had a woman refuse to dance with me because "She wasn't that way." Well, honey, if I was, I sure in the heck wouldn't hit on you.

Lastly, the partner who continues to count out the tempo after you have found it again, after briefly losing it. >
All and all, dance people tend to be pretty polite, it's only the dance snobs, and meat-marketers that you have to watch out for.
-TheDitz