Many liberal people know the value of two parent families over single parents. But again the tail, the far left, is wagging the dog.
The far left hates America and the old-fashioned two parent family with a father and a mother. The radical feminists hated men, so they had to promulgate the myth that men are not needed to raise children. I read an appreciable amount of feminist literature back in my college days, and the hatred of men and father and mother families was palpable.
Of course, two parent families (father and mother) are far superior to single parent families on average. But some people (the usual suspects) don't want to acknowledge that for ideological reasons.