There are two prerequisites for calling a woman a slut:
  1. You don't like her, and
  2. She has sex more often than you do.
The term 'slut' is usually directed at a woman the labeller does not want to respect. It implies that she has sex more often because her perception of self is lower than that of the person doing the name-calling, and therefore it makes less of a difference whom she sleeps with.

The self-esteem issue is sometimes true, but it is not helped any with the 'slut' label. If anything, the label may aggravate the problem.