A little bit of history:

We used to have an "old maid's tax" from 1930s to 1975. If you were still unmarried at 24, your income tax rate would be 3-6% higher than that of a married person with kids. If you were married but didn't have kids, your tax rate would be slightly higher, too. Medical reasons (e.g. infertility) wouldn't save you from the tax, either.

The obvious reason for the tax was to "encourage" people to marry young and have kids. Imagine the shitstorm if someone proposed such a tax today.