I'mm not so sure. I don't know if they mean single mothers too, but there is a lot of emotional development in having a family and raising children.
IF you get to raise your children.
The thing everyone is missing is the risks involved in getting married and having kids. Your wife at any point can cheat, leave, and take everything including the kids. Do you want to be a wage slave the rest of your life? Do you want to join the millions of single fathers who get destroyed by broken, bias, and corrupt family courts?
If Conservatives and Republicans want to address this issue, start by fixing family courts, eliminating alimony, supporting shared parenting. Start removing the risks to marriage and reducing the number of single parent homes. Address the cultural and legal issues surrounding this.