Høiby, 29, has been charged with four counts of rape amid a total of 38 charges in a case that has drawn massive international attention.
On the second day of the trial in Oslo, the court heard testimony from a woman who, according to the indictment, was penetrated by Høiby with a finger while she slept during a party at the royal residence of Norway's crown prince and princess.
The newspaper Verdens Gang cited the alleged victim describing her "betrayal and shock" following the incident.
"At first, I didn't believe it," she reportedly said. "I couldn't understand that Marius could do something like that to me."
The woman also said she had never previously experienced a blackout of that kind.
"Maybe I was given something that I didn't consume myself," the newspaper quoted her as saying.
Høiby is scheduled to respond to the allegations later on Wednesday.
Høiby's defence lawyer, Ellen Holager Andenæs, told Verdens Gang on Wednesday: "It is his wish to explain himself."
On the first day of the trial on Tuesday, Høiby only briefly commented on the accusations against him, denying the rape and serious sexual abuse of several women.
He admitted other offences, including cases of domestic violence, damage to property and drug abuse, along with traffic offences.
Another woman was allegedly raped in her sleep during a stay on the Lofoten Islands in Norway. She is said to have woken up during the attack. The two are said to have had consensual sex previously.
The media interest in the case is immense. Some 200 journalists were present to follow the first day of the trial.
Shortly before the trial, Høiby, who was arrested on Sunday and remanded in custody on new charges of assault, making threats with a knife and violating a restraining order.
Mette-Marit's son was then remanded in custody for four weeks. "This is of course not at all good for the case," defence lawyer Andenæs said.
Høiby's indictment centres on four alleged rapes between 2018 and November 2024; alleged violence and threats against a former partner between the summer of 2022 and the autumn of 2023; and two alleged acts of violence against a subsequent partner, along with violations of a restraining order.
It was expanded in January, when Høiby was charged with six more offences, including possession and delivery of large quantities of marijuana and further restraining order violations.
Mette-Marit's son from a previous relationship before she married Crown Prince Haakon, Høiby is not an official member of the Norwegian royal household.
Haakon and Mette-Marit have said they will not attending the trial and do not wish to comment on it.
Mette-Marit had originally planned to travel at the start of the trial but has since postponed her private travel plans, a spokeswoman for the royal family told DPA, without providing further details.
The Norwegian royal family is facing another controversy following revelations that the crown princess's name appears hundreds of times in the recently published Jeffrey Epstein files.
She is said to have exchanged private emails with the convicted US sex offender over several years.
with DPA and Reuters
--------------
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
18°C