The couple should be allocated some large number of days which they can choose to divide between them in any way they want. This is how it works in many scananavian countries, and I think a similar system was recently introduced in the UK too.
In law, the government can either follow a trend, or change the status quo. In this case, changing the status quo and forcing the fathers to take some responsabilities of mothers is the only way. A sexist society doesn't change by itself because the men in power doesn't want it to.
http://www.economist.com/blogs/economist-explains/2014/07/ec...