Throughout history there were single parents. Think of all the wars and the men that died in them. They left a lot of widows and half-orphans. 

And being a single parent doesn't mean being alone in raising a child, that nobody but one person cares for the child. People get older so there are more grandparents living than they used to. With more and more people having no children they have the time to help raising children of their siblings. 

Having two parents doesn't mean having two care givers. How many fathers come home from work late when their children are nearly asleep. Absent parents aren't better than none. Abusive parents aren't better than none. 

"It takes a village to raise a child" - that's really true.