> much more if they are married to a zero-income spouse

In France it's the same, but here's a major difference once you add kids to the equation:

Each kid of a couple counts the same as "0.5 zero-income spouse", up to 3 kids. After the 3rd kid (or if the kid has handicap), they count the same as "1 full zero-income spouse"

So for a couple with one spouse with a salary of X , one spouse without job, and 2 kids, the taxes would be calculated as if there was 3 persons making X/3 each, instead of 2 people making X/2 in the US. Given that the tax brackets are even more progressive than in the US, the impact seem huge.