Unless on is wealthy and can afford to pay for help and daycare it's probably not the best idea to (choose) being a single parent. However sometimes couples get divorced and one parent is left with being the "primary caregiver". However if the other parent is (truly) active in the child's life then he or she views themselves as having two parents! In my opinion it is only when one parent has nothing or very little to do with the upbringing of a child that is when you can say you were raised by a single parent.
Today there are so many couples who (live together) but have never married that choose to have kids. Legally these couples are "single" and free to marry anyone they choose. Single parent use to refer to one's "marital status" as in divorced, never married, or widow/widower who was left to raise a child (alone).
Today many people consider themselves to be single parents even if they are getting help from the other parent. In their mind if both parents don't "live together" the child only has (one) parent. While it's a fact the man or woman may not have a spouse or is living with a lover this does not mean the child has only (one) parent.
Having said that if one truly is playing both mother and father roles it's not the ideal situation for most children.