A judge has questioned why underage boys engaging in sex with underage girls are often prosecuted for the crime when their girlfriends are not.

Judge Paul Muscat has expressed his concern during sentencing submissions for a young man who pleaded guilty to have an unlawful sexual relationship with his underage girlfriend in South Australia.

The Adelaide District Court heard their relationship and sexual activity were consensual and started when the teenage boy was underage.

But the court heard the pair had intercourse after the boy turned 18 and when he broke up with the girl her mother found out and went to police.

Judge Muscat said the girl also would be liable for prosecution for sexual activity that occurred when the boy was underage, but he never had seen such a case in the Youth Court.

"If this activity was brought to the attention of the police, in her own statement of what happened, putting aside the acts of intercourse, she would have to be charged along with him," the judge said.

"I have never seen that in the Youth Court and I always wondered why only the boy is charged, but there you go."

Judge unlikely to record conviction

Judge Muscat said it was not the kind of case he would consider a custodial sentence for, which usually would be a penalty for an adult having a sexual relationship with a child.

"It seems to me this is a particularly obvious case of boyfriend/girlfriend situation which he eventually called off and she's upset by it and one thing leads to another," he said.

"Before one knows it, he's charged with the crimes and these crimes have now devastated her whole life."

He said the case was not the sort of thing that criminal courts should be dealing with.

"He admitted his guilt, he was just 18. From what I read of the complainant's mother's declaration, he was immature," he said.

The court heard the girl claimed not all the sexual activity was consensual, as she was coerced into some of it.

But the boy's lawyer said that allegation was strongly denied and the prosecution had not charged him with rape.

Judge Muscat said he would likely put the boy on a simple bond without recording a conviction when he delivered his sentence in June.