Traditionally, a dance will have equal numbers of men and women or an excess of men. This was often a result of the fact that many dances were guest proportion planned (part of the etiqutte of throwing a party), or was a dance put on for the benefit of a group or trade that was traditionally male run (such as a military or a banker's trade ball).
The rules on who-asks-who goes as follows:
The reasoning is to prevent hurt feelings among those who are most likely to be hurt during the course of the night (that is, those who are in the majority since there is not enough of the minority to "go around").
To demonstrate the reasoning of this rule, consider the following: If there are more women than men, and the men are asking the women to dance, inevitably there will be a number of women who must sit out the dance simply because there is nobody left to dance with. This leads to a "Why was I not asked to dance?" question on among some of those who missed out, leading to a sense of rejection, of being less than preferred, much like the kid who is always being picked last for the sports team. On the other hand, if in the same situation, the women were asking the men, as the rules would dictate for this situation, then all the men will be asked (none will be left behind), and those women who did not manage to get to a man first will be ready to ask one of the newly freed men after the current song ends. The women who did not get a partner in this case did not get a partner because they themselves were not quick enough to grab an available man rather than because they had been overlooked (or "picked last").
If you are a woman, do not believe that "Its the man's responsibility to ask me to dance."
Regardless of who is in the majority, if you really want to dance with someone, do ask.
A dance is a social event where everyone is on equal standing.
A dance is not a dating service (unless it has been specifically organized as one); just because you
are dancing with someone of the opposite sex does not make the dance a date, convey any interest by either person,
or imply any sexual interest between the parties involved.
Because of the same fact, it is also acceptable for two men or two women to dance a dance together.
In this same vain, it should be noted that some dances hosted by religious groups still do not permit a man and a woman to dance together. Instead, dancing is only permitted between people of the same sex. On the other hand, other religious groups, most particularly American based Christian groups, frown on people of the same sex dancing together.
Force of habit, personal attraction, dancing capabilities all can influence who someone would like to dance with. However, this will often cause someone to choose the same few individuals to dance with all the time. This is not helpful to the social dynamics of a dance. Because of this, etiquette outlines your choice in partners to ensure a diversity of partnerships, and to give everyone a chance to dance.
Generally, do not dance with the same person more than twice in a row. Do not monopolise a single person all night. The biggest violation of these two rules is the habit of people to dance most of the night with their escort (the person who you came with).
Only the first and the last dance of the night should be danced with one's escort. The exception to this is when their are very few people at the dance to choose from making this less resonable.
People tend to dance with others at their own level, but excluding partners based on their level is not acceptable. Do not continually seek the most skilled partners, this is against the spirit of social dancing. It is advised that better dancers should ask beginners to dance. Today's beginners will be tomorrows good dancers. Not only does this help the social dynamics of a dance, it also helps the better dancer.