A judge has struck down a mandatory minimum jail term of one year for adults who engage in sexual activity with minors, a ruling that comes as the Liberal government mulls what it should do about obligatory sentences for sex crimes against children.

Justice Maria Linhares de Sousa of the Ontario Superior Court of Justice gave a sentence of seven months in jail and two years' probation to 50-year-old man found guilty of sexual interference for fondling the breasts of a 15-year-old girl. In her ruling, the judge called the law that established the mandatory minimum for the offence unconstitutional, saying one year would be grossly excessive in "reasonable hypothetical" cases, although not in the one currently before her.

The ruling is binding only on lower-court judges in Ontario. Federal Justice Minister Jody Wilson-Raybould told The Globe and Mail last month that she intends soon to cut widespread use of mandatory minimum sentences – a centrepiece of the former Conservative government's crime agenda. The Conservatives passed 60 mandatory minimum sentences, mostly involving drugs, guns and sexual offences. The mandatory minimum for sexual interference became law in 2012.

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Whatever the government does with minimum sentences for sex crimes against children, the ruling shows judges will not simply accept them quietly. If the government wishes to keep those sentences, it may have to fight for them in court.

Constance Backhouse, who holds the Chair in Sexual Assault Legislation at the University of Ottawa law school, said in an interview the ruling "adds more fuel" as the Liberals consider which mandatory jail terms to roll back.

"Certainly, people who are concerned about sexual assault are aware that the vast majority of these incidents go unaddressed in the criminal law," she said. "They're not reported, they're not prosecuted, there are no convictions. And when there are convictions, frequently the sentences are out of line with other sentences in the criminal-justice system."

But she said she opposes mandatory minimums because they prevent judges from taking into account a case's intricacies.

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A spokeswoman for Ms. Wilson-Raybould pointed to a speech last month in which the Justice Minister said reforming mandatory minimum penalties is a priority for her, although she did not say which ones would fall.

Hilla Kerner, a spokeswoman for the Vancouver Rape Relief and Women's Shelter, said in an interview that she supports strong sentences for sex crimes against children, but "we think judges should have the discretion and the autonomy to decide on appropriate sentences. The way to go is not to tie judges' hands but to make sure they understand the nature of these crimes and how men use their power to attack women and children." The Ontario Attorney General's Ministry declined to comment while it might still appeal the case.

The reference to reasonable hypothetical cases is controversial even at the Supreme Court, which established the concept. In a 2015 case, R v Nur, the court voted 6-3 to strike down a three-year minimum for illegal gun possession based on hypothetical cases. The dissenters said repeatedly that the hypothetical examples were out of step with "experience and common sense."

In the sexual-interference case, the man fondled the girl once and stopped when asked during a night in which she slept in his bed, as his daughter and her friends had done before with no reported incident. The man had no criminal record, but was in a trust relationship with the girl, an aggravating factor in sentencing. The girl was raised without a father, and said she had suffered deeply – another aggravating factor – because the man had been like a father to her.

In her constitutional ruling, Justice de Sousa said she was not sure the man, a registered nurse whose name is not being made public in this story to protect the victim's identity, was truly remorseful. The judge based the sentence of seven months on previous cases, including in the period predating the mandatory minimum, and according to the man's lawyer, Mark Ertel of Ottawa, a psychological report that he was at low risk to re-offend and a good candidate for rehabilitation. She ultimately accepted his expression of remorse, she said.

Lisa Kerr, a professor at Queen's University law school, called the ruling "yet another indication of the willingness of the Canadian legal system to closely protect judicial discretion to impose a proportionate sentence in all cases."

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Mr. Ertel explained in an interview that courts used to grant "constitutional exemptions" where minimums were unfair, but the Supreme Court has ruled this out, saying a law is either fair or not.

The Conservatives' justice critic, MP Rob Nicholson, could not be reached for comment.