From Wiki:
The use of the term "real" implies that the adoptive family is artificial, and is not as descriptive.
From "Adoptive Parents = Real Parents":
"They're not your real parents!"
The little girl's words were not meant to hurt. At 8, she was struggling to understand her own adoption. But our adopted son was hearing these w ords for the first time, at the age of 5. And he was devastated.
For weeks, Mark could not bring himself to share the incident, nor his pain, with us. What terrible thing might happen if he did? His behavior showed he was troubled, but even when he came to our room at night complaining of monsters, he could not speak of his underlying fears.
One evening, as he lay with his head on my lap, with my hand stroking his forehead, Mark broke into sobs and burst out with his terrible discovery: "Becky says you're not my real parents!"
I hugged and reassured him. He had known for a long time that he was adopted and that this meant his "first parents" could not take care of him. He knew that we had become his "forever parents" because we were able to provide food and toys and clothes, and because we very much wanted a little boy like him. As a social worker in adoption, I had read all about "telling" and had thought that our explanations had covered all the bases.
But we had never dealt with the question of "real" parentage. It simply had not occurred to us that other children would openly assert that "forever parents" were not real parents. Yet this has happened to Mark at least three times in the years he has been in school.
Mark knows what to answer now: "Yes, they are my real parents," he will insist, "because they are the ones who are bringing me up." If the other child persists, he will say, "You're confused about that!"