The teenage girl had been beaten severely with a wooden stick and a ruler against buttocks, thighs and genitals.
The man was previously acquitted as the girl had consented to the sexual role play.
But the prosecutor argued that it's not possible to consent to aggravated assault and therefore appealed the verdict.
Although the court found it beyond doubt that the teen had been assaulted by the man, it also judged that the girl had consented to the violence, based on evidence given by the man.
During the trial, the girl partly denied having given her consent, but the court judged her testimony as only having ”limited credence”.
The girl's defence lawyer, Kerstin Hedelin, told news agency TT that the her client's self-harming behaviour in the past should have been considered.
”It is important that one actually see her as the very vulnerable girl that she was,” she said to TT.
Hedelin also refers to research that shows that it's not uncommon for girls with self-harming behaviour to let themselves be sexually abused as another way of harming themselves.
”When cutting yourself doesn't have the same effect any more they revert to this,” she said.
Prosecutor Ulrika Rogland told TT that she had hoped for a more conclusive ruling. According to her, the court brushed aside the complications of the particular case.
”They discuss the case as if it is a question of two equal partners and as if he basically just did what she wanted. They ignore the problem that she was much younger, that they had never met before, and that the situation perhaps required more from the adult,” she told TT.
She added that although the court mentions the girl's self harming behaviour, they miss how ”severe” it was.
In the earlier trial she was not present but in the appeals court she appeared and showed the court the scars she had from where she had cut herself.
The defence lawyer of the 33-year-old, Bo Petersson, told TT that this means the court of appeal has ruled that it's acceptable to practice this kind of sexuality.
According to Petersson, what went on between his client and the girl falls within what is generally characterised as ”normal BDSM-sex”. On the condition, he added, that there is consent.
But Petersson doesn't think that the case will set a precedent for the limitations of violent sexual behaviour in court.
”In some way it seems self evident that those with this inclination to practice their sexuality even if it means bumps and bruises,” he said to TT.
When it comes to assault the ruling says that it is ”normally not a criminal offence to administer pain or modest injury with the consent of the other”. The violence the man put the girl through is therefore not deemed indefensible.
As the man was acquitted, the girl will not be paid any damages. The court was unanimous in its decision.