Legal Question in Criminal Law in California

No.

Sex between an adult and a minor is never legal. The boyfriend you describe would be committing a felony. The parents would arguably also be committing at least one crime. They would also be giving the government a good reason to take the daughter (and any other children) away from them.

Mr. Hoffman is right on the money. The fifteen year old girl has an excuse because she's still a child, but all three adults -- her parents and her boyfriend -- are idiots.

If she becomes pregnant, the child's DNA test would prove he was the father, making a prosecution pretty easy, even if nobody wanted to testify.

Do the terms "statutory rape" or "child neglect" come to mind? Both are applicable to this as felonies.