A lawyer says sex offender laws are jeopardising the lives of young South Australians who are convicted of underage sex.

The law requires that anyone convicted of a child-sex offence be put on the sex offenders register.

But Adelaide lawyer David Stokes says the law is also catching teenagers who are not dangerous sexual predators.

A District Court judge has expressed concern about sentencing Mr Stokes' client, who was 19 when he engaged in sexual activity with a 14-year-old girl.

The tradesman, now 21, was given a suspended prison term for indecent assault and unlawful sexual intercourse.

The court heard the sex was consensual, but as the girl was under 17 she was legally unable to consent.

Mr Stokes has written to SA Attorney-General John Rau asking him to amend the laws to allow judges to decide which offenders should be placed on the register.

Mr Stokes said the laws were socially unjust and were unfair for the small percentage of sex offenders facing the circumstances his client faced.

"He's going to face massive restrictions on his life because he had a consensual bit of mucking around with a girl who was underage. He was 19 at the time, it's just silly and it's unfair," he said.

"At the moment it's like drift-net fishing, it will catch everybody.

"It's inappropriate to have this kind of draconian legislation affecting young people in these circumstances."

Mr Stokes hopes the Attorney-General will see the sense in making changes.

"The court needs discretion in appropriate cases not to have someone on the register because at the moment the courts aren't even allowed, as I said, to take it into account. They are destroying people's lives," he said.

"I'm just hoping the Attorney might be able for at least five minutes to avoid looking at problems within his own party and the parliament and concentrate on these sorts of social justice issues at the same time. I'm sure he can, he's a good lawyer."

Common issue

Mr Stokes said it was not uncommon for people over 17 to have sex with underage partners, but only some cases reached the courts.

He said it was not the first such case he had encountered.

"It can affect their employment and their lives for one-off or two-off consensual acts of intercourse, which thousands of young people are doing on a weekly basis in Adelaide. It's nonsense," he said.

"The consequences for employment may be quite dramatic; as a sub-contractor or contractor where much of your work is say to do with building schools and kindergartens and places like that, you can't work there."

Sentencing judge Michael Boylan also expressed concern, but said the law did not allow him to take account of the impact on the young tradesman of being on the sex offenders register.

"I must say it is unfortunate to find a young man in your position facing sentencing for this type of offending," he said.

Judge Boylan said the conduct was serious and the law was there to protect young people from those older.

But he said that it was most unfair the man would become a registered sex offender.

"I am not permitted to take that into account and I make it clear that I have not done that," he said.

The man was given an 18-month good behaviour bond and is now subject to monitoring and controls under the sex offender register system.