@melgillman thanks for including the source. Far too many people don't when they share information such as this.
I've started reading it and one thing that I noticed was that the reason seems to be that women simply work less after having their first child. That would explain why the graph trajectory matches the men's (but lower) again after a few years.
I know Sweden are changing their child leave policies in order to equalise this. I don't know if Denmark is doing the same.