How do you know when a man in the office becomes a father? If you institute "mandatory paternity leave" then he just won't tell you. It's already illegal to ask about marital status during a job interview. Are you suggesting we change that, too? Then you risk further reducing the number of marriages in society.

The whole problem with paternity leave is that there's a financial disincentive for companies to offer it and there's a financial disincentive for male employees to accept it.