An Adelaide judge has questioned why a man found with pornographic images of his underage girlfriend was prosecuted, when the couple were in a consensual relationship.

The 21-year-old man, who ABC News has chosen not to name to avoid identifying his girlfriend, pleaded guilty to offences including possessing child pornography, producing child pornography and making a communication intending to procure a child to engage in sexual activity.

The District Court heard that during a search of his car and home, police found 12 pornographic images on his mobile phone, each showing his girlfriend who was at the time two months short of her 17th birthday, and the age of consent.

The court heard the images were explicit, but showed the girl semi-clad and nothing more.

Only three of the images were taken by the man, the rest had been sent to him by the girl.

In sentencing, Judge Paul Rice said it was a case at the lower end of the range of seriousness for offending of this type.

"I note that the names and nature of the offences in this case sound a lot more serious than they are on the facts," Judge Rice said.

"I want to make some general comments about the offending.

"It is true that you were older and probably more mature than your girlfriend.

"The law would suggest you should have been shielding her from her immature inclinations."

Judge Rice questioned whether it was right to prosecute the man.

"There is a lot to be said for the view that this would have been an appropriate case for the director to decline to prosecute," he said.

"It was a completely consensual relationship known and embraced by your parents and I expect hers.

"You were just 21 years without prior offending. She was almost 17 years. You were doing no more than I expect thousands of young people do on a weekly basis even at your ages."

Judge Rice ordered no conviction be recorded, but placed the man on a $500 one-year good behaviour bond.

The court heard there was some lack of clarity over the laws surrounding whether the man would still be placed on the Child Sex Offenders Register, with some suggestion the discretion now lies with the Police Commissioner.

"If I had the power to direct that your name not appear on the Child Sex Offenders Register on the information before me, I would do so without hesitation," Judge Rice said.

"The courts should have some discretion in this case and others like it.

"The legislation is very important and necessary, but as currently worded, it is also very blunt and inflexible with no element of discretion for very minor cases such as yours."