In a perfectly fair employment world, remuneration will be linked directly to the contribution the employee makes to the firm. Using strictly objective criteria, devoid of what economists term 'taste discrimination', this can be quantified by an equation that rates education, experience and a third factor which for brevity we can call 'employability'. A man and a woman the same age with exactly the same 2:1 from the same university in the same year and whose 'employability' is exactly equal who are working for the same firm in the same job would therefore be remunerated equally just as long as they both have equal experience in the workplace.
If women, on average, spend less time in work than their male counterparts, a perfectly equal system will reward them less. Women taking time off for maternity will therefore be rewarded less on average and across the board. The facts bear this out; of the 18% gap in earnings, around 4% has been attributed to 'taste discrimination', but the balance is due to women having lower aggregate experience.
If you think this is unfair, there are two ways to deal with it. Either you can force firms to pay less efficient staff the same as more efficient staff - grossly unfair - or encourage fathers to take childcare responsibility and take time out of the workplace as women do. The latter is the option we've chosen. It's fair, it keeps objective reward systems intact, and the aggregate time lost to business remains broadly the same. Sure, it will take time to erode that gender pay difference, but we can monitor it.
Of course it assumes that man and women who have children come in working couples - something deeply unattractive to some gobby interest groups. And if working couples decide that it's the woman who takes a few years off and the man who continues in work, we must respect that also; just as long as the opportunity is there for it to be the other way around is enough to ensure we have a fair and equal system.