As I have pointed out earlier, I am not judging. If they have the economic means, and the ability to substitute for a dad, then all the power to them. Unfortunately, statistics don't back that up. 75 percent of men in jail in the US come from single mother families (therefore dads are needed for income and as role models). A majority of kids from these families don't go to college or university. They end up in minimum wage jobs.

For the average child to be as or more successful than his parents, he needs a two parent home. 

As for the cohabiting parents, a majority of those relationships does not last. Again, statistics.

Finally, single parent families are a major reason that child poverty in the US is around 40 percent. Social welfare in Europe and Canada ensure that these kids have a better school, and a much better chance at going to a post secondary institution.