@tfus,
Nothingness has the property of having no properties. Hence its a contradiction, hence many reject it. Parmenides, a Pre-Socratic Philosopher, and influence on Plato, is an interesting example.
Unborn people is not a non-existant concept, its an imaginary or fictional concept. In the strictest sense, anything you can think of exists, non-existance is outside of that set. Unborn people have the qualities of being people and being unborn, and unless you add additional criteria, these are the only qualities of them. As for what unborn and people entail, that's up for a lot of debate.
There are things which we would not call "real", but do exist. Superman is an example of this. If you jump off a building, you cannot reasonably expect someone to fly down and prevent you from dying. Still, that does not make superman non-existant, he certianly has defined qualities.