A woman cannot give advance consent to sexual activity while unconscious, the Supreme Court of Canada ruled Friday.

The decision restores the conviction of an Ottawa man who regularly practised consensual erotic asphyxiation with his longtime girlfriend.

The case goes back to a particular episode in 2007 when the woman, who cannot be named because of a publication ban, complained to police about what her partner did to her after she passed out. At trial, the man was found guilty of sexual assault but his conviction was overturned on appeal.

On Friday, in a 6-3 decision, the country's top court restored the conviction. Writing for the majority, Chief Justice Beverley McLachlin said consent ends once someone is unconscious or asleep.

"If the complainant is unconscious during the sexual activity, she has no real way of knowing what happened and whether her partner exceeded the bounds of her consent," the ruling said.

The definition of consent is an ongoing state of mind where individuals can ask their partner to stop, McLachlin wrote.

"Any sexual activity with an individual who is incapable of consciously evaluating whether she is consenting is therefore not consensual within the meaning of the Criminal Code," she wrote.

Concerned about freedom of choice

Three Supreme Court judges disagreed. In the dissenting opinion written by Justice Morris Fish, the judges said Friday's ruling would deprive women "of their freedom to engage by choice in sexual adventures that involve no proven harm to them or to others."

They also expressed concern about the criminalization of normal sexual relations between spouses.

"The approach advocated by the Chief Justice would also result in the criminalization of a broad range of conduct that Parliament cannot have intended to capture in its definition of the offense of sexual assault. Notably, it would criminalize kissing or caressing a sleeping partner, however gently and affectionately."

Elizabeth Sheehy, a lawyer for the Women's Legal Education and Action Fund, which intervened in this case, dismissed the dissenting opinion. She called the majority ruling a major victory for women.

"The most important message that the court is communicating is that unconscious women are not sexually available," she said.

The Ottawa man served 18 months in jail after his conviction in 2008 on the sexual assault charge.