The rationale is that apparently due to rampant adultery a lot more fathers than previously thought are raising another man's kids without knowing it (apparent

The rationale is that apparently due to rampant adultery a lot more fathers than previously thought are raising another man's kids without knowing it (apparently about 10% last time I read about it, also on HN) and since raising kids to adulthood is very expensive and so if the men could easily find out their kids are not their own, then the man can legally opt-out of that financial responsibility then the French state/taxpayer has to pick up the tab for the mother's adultery. So if you make it nearly impossible for men to ever find out, they'll be legally bound to pay for the upbringing of whoever their partner gives birth to, reducing the financial liability of the state. But the official reason from the state is something along the lines of "we don't want to put kids/families through unnecessary stress". Basically men have to be cash cows regardless.