The pay gap is around 15-25% depending on the country (in western countries), this is accurate. The problem is that a lot of people think this means a woman is paid 20% less for the same work, which is simply not the case and leads to a lot of anger and arguments.
This statistic simply compares the average earnings of full time working men vs full time working women. It does not factor any of the following:
-Men work longer hours, around 4 per week on average (10%)
-Men choose to work in higher skilled and higher paid professions eg engineering vs childcare
-Men are more likely to move geographically to follow higher paying work
-Men are more likely to take on jobs with a safety risk premium such as mining and construction

• Studies find women to be less aggressive negotiators. This means that in lower positions with more standardised pay packets the gap is very small whereas in the most senior positions where eligible candidates are in short supply the gap widens.
  -Women often take career gaps to care for family which lowers their experience relative to male counterparts

Studies usually estimate that the discrimination portion of the gap, ie the part of the gap that is due to sexism is around 1-5%. Although some studies are beginning to find that women are now paid more for the same work.

Equal work for Equal pay should be the goal, and freedom of choice.