If you are asked to dance a song before you can ask (or get asked by) your desired partner, that's the luck of the draw. The choices are to dance it with whomever asked first, or to sit out the dance.
The only reasons for declining a dance is:
When declining a dance, one should ask for a later dance instead: "No, thank you, I'm taking a break. Would you like to do another dance later?" However, it is improper to book many dances ahead, or too often.
Especially for beginners and shy individuals, being declined can be difficult, and may discourage them from social dancing. Dance etiquette requires that one should avoid declining a dance under almost all circumstances. It is forbidden to refuse on the basis of preferring to dance with someone else.
There are some people who consistantly violate the rules of conduct, who are rough, or dangerous on the floor, rude,
or otherwise a serious problem.
Etiquette allows an out from the rules so that one does not become oppressed by the inconsiderate.
If such a situation arrises one is allowed to say:
"No, thank you." without further explanation.
This option should be used with great restraint, and only in the case of repeat offenders.