Do you blank people without even knowing it? Blanking is my word for avoiding being asked to dance with body language, it's as simple as avoiding eye contact, looking away or turning away when someone walks towards you.
I notice this more in some situations than others, if I'm unknown in an area I get it more, if I've already danced with everyone I get it more, if there is a shortage of guys I get it much less! Worst of all, I do it to other people, often without trying or thinking about it.
Is it ok? Yes and no, it's slightly nicer than saying 'no thanks', and kinda reasonable if you are walking off the dance floor for some urgent reason..., But mostly, it's a bad thing to do and it's spreading a bad vibe through the room. If you are at a dance evening, then you should be open to dancing with everyone (creepy or dangerous people excluded), so hey, lets try smiling at everyone, and saying 'Yes' with our eyes, and then yes with enthusiasm when asked to dance, no matter how much you'd rather be dancing with someone else!