We forget what the real cause of single motherhood.
It isn't defending this type of marriage over another type of marriage. The State interest in Marriage is social and financial. Single motherhood puts a financial burden on society to care for the abandoned mother and orphaned children.
This is where the government interest came in. They required men to care for their families.
However, since FDR there have been more and more government programs created and funded to support single mothers. This has made the government the new head of the household. Instead of creating citizens that are independent and self reliant, the government has created a dependent class.
The original government interest in marriage is gone.