The authors of this regulation probably assumed that married couples would not be induced to split just to collect money, and that single moms would stay chaste in order to collect, and thus not have more out-of-wedlock children. So they set up a law that incentivized behavior they were trying to avoid. It's like the Marriage Penalty ( https://en.wikipedia.org/wiki/Marriage_penalty ) except worse.

So it probably wasn't a result of maliciousness but rather a lack of imagination and empathy (aka stupidity).

It sounds progressive at the time.

Men worked, if one wasn't part of the home then women needed more support.

poor women worked for decades prior to 1968

I'm guessing the justification was intentional maliciousness.