A number of people in this thread appear not to be reading the article. The title sounded pretty ridiculous to me, so I knew it couldn't be what it sounded like. It's not. The girl was not raped, but under Swedish law the man was accused of comitting rape by having sex with an underage girl. From the article:

Under Swedish law, having intercourse with someone under the age of 15 is classified as ‘child rape’, but both Västmanland District Court and Sweden’s Court of Appeal have now thrown out the girl’s case, because the legislation also states that a defendant must “know” or have “reasonable grounds to believe” that the child is under age.

They had consensual sex, she later regretted it and tried to press charges. Her attorney claimed that the law allowed it to be prosecuted as rape (presumably the law considers children under the age of consent unable to give informed consent, thus even consensual sex is rape) However it also states that the perpetrator must have grounds to believe the other party is underage.

In this case the court is saying that he had every reason to believe she was over the age of consent, she agreed to have sex with him, he therefore didn't break the law. Whether or not it's a good idea to have sex with someone borderline age of consent isn't relevant here.

"The Supreme Court has been very restrictive when it comes to applying legal sexual abuse clauses to cases involving young teenagers," she told TT, adding that "less serious crime classifications" than "child rape" had often ended up being used in other similar cases.

The rape law in question allows that the accused rapist may not be aware the other party is underage. A less severe crime may have gotten a conviction. The law here is reluctant to sentence someone to harsh sentences in a borderline case.