Girls who have sex at an early age are at slightly greater risk than their peers for feeling depressed, a new study has found. But their self-esteem suffers only if the sex occurs outside a romantic relationship. ...I think she is trying to say that it is psychologically healthy for young girls to have sexual relations as long as everyone else is doing it.
First sex was associated with a decrease in self-esteem, but only among girls who were younger than the average age and not in a romantic relationship. Girls who had sex at the average age or later had no increased risk for depressive symptoms compared with those who had not had sex. ...
"I think this is a good study," Dr. Lindberg said. "I'm struck at how much you have to recognize that teen sexual behavior occurs in a social context."
"One of the big findings here," Dr. Lindberg continued, "is that it's age norms and not the actual age that matters, and these norms vary depending on what group in society you’re in."
The trouble with these psychologicsl studies is that all they can really do is tell you about behavior that is outside social norms. They can't tell whether the behavior is good or bad. I am not sure they can even say that early sex causes low self-esteem; maybe young girls with low self-esteem are more likely to have casual sexual relations.