There is legislation which means a company can NOT offer a man more for doing the same job as a woman, however there are a number of reasons why men end up with more than women.
1 Jobs for the boys. A man's salary will often be enhanced with things like cars, overnight stays on golfing trips, etc so that on paper, while the salary is the same, the overall package is worth more.
2 More women take career breaks than men. It's women who take the lion's share of parental leave when they have children and as the law was only changed recently to allow men to do this, it will be a while before it is seen as the norm.
3 Women do the majority of child care. Even once a woman goes back to work after maternity leave, it is more likely to be her that goes part time to accommodate childcare. If she goes back to the same job, that's fine as it means her original salary will simply be reduced pro-rata according to the hours she does. But if her employers don't allow part time, rather than the father requesting part time, it's more likely that the woman will look for alternative employment which leads on to number 3.
4 Women are over-represented in lower paid areas of work. They make up roughly 50% of the work force (not exactly but near as damn it) however 75% of working women work in some of the lowest paid occupations termed as "the 5 Cs": catering, cleaning, caring, cashiering, clerical. These are often taken up as the location is more convenient to schools, swapping shifts, etc.
5 "Women's work" is under-valued. There is evidence that could suggest that over the years, since women joined the workforce, jobs that have become associated with women have been devalued. For example, 50 years ago, Personnel Managers were probably men and would have commanded a similar salary to any other manager in a company. Nowadays, HR tends to attract a higher proportion of women than men and you could argue that salaries have, comparably speaking, reduced.
A lot of the reasons for the discrepancies are cultural. You've got books such as "Men are from Mars, Women are from Venus" and "Why Men don't listen and women can't read maps" reinforcing gender stereotypes and even advising what kind of job to do. If you follow the theories peddled in these, men are "better" at spatial awareness, concentrating on single projects, more aggressive; women are better at doing lots of little things all at the same time, doing jobs that involve talking to people and nurturing roles. Women in general are less likely to become engineers, architects, scientists and more likely to become nurses, nursery teachers, customer service reps, etc.
It's not all bad though a lot of these issues were much worse 10 years ago before the introduction of the new paternity rights so we're heading in the right direction. 