A man has received a suspended jail term for having sex with a 13-year-old girl who wanted to work as a prostitute to obtain money for her family.

Leslie Kenneth Madden, 24, met the girl through an account she created on a dating website and she told him of her prostitution plans.

"Indeed, at one stage she was hoping you would not be a customer but her `protector'," said Judge Michael Boylan when sentencing Madden in South Australia's District Court.

Madden, from Adelaide's Windsor Gardens, pleaded guilty to one count of unlawful sexual intercourse, on the basis that he had not known she was only 13 and believed she was about 17.

Describing the circumstances of the offence as unusual, the judge said some warmth of feeling seemed to have developed between the pair, mainly through phone calls.

"You were not keen to begin a relationship with a prostitute," the judge said.

But in August last year, she came to his house, where they had sex and he paid her $150.

"After the sexual encounter, according to your victim, she told you that she was only 13."

The judge said Madden's offending had caused great damage - to Madden through now being listed as a sex offender, and to the girl who felt responsible for Madden being before the court.

She admitted she had gone out of her way to look older and now felt she had been unfair to him.

"A girl of her age should not be in a position where she feels that way," the judge said.

"You were much older than she, certainly old enough to know that younger women should be protected from themselves.

"In all of the circumstances surrounding your communications with her before your offending, you should have been on your guard that she may have been younger than she appeared and, indeed, behaved."

The judge accepted Madden was very sorry and was extremely unlikely to reoffend.

He sentenced Madden to two years and three months, but suspended the term and placed him on a two-year good-behaviour bond.