A 19-year-old Canberra man who sexually assaulted his long-term girlfriend during a fight has walked away with a good behaviour order, despite the fact the woman he abused received a suspended jail sentence for the same incident.

The ACT Supreme Court heard on Monday that the man, who cannot be named for legal reasons, had been dating the woman for five years in June 2017, when the pair began to struggle after an argument.

During the fight, the man grabbed her by the neck and placed a pillow over her head, while she bit him and pulled on his genitals in order to cause him pain.

He then pushed the woman onto the ground, before forcibly penetrating her with his finger.

The man later pleaded guilty to sexual intercourse without consent and common assault.

Woman and man suffered 'equality of injury'

While handing down the sentence, Justice Michael Elkaim said the facts behind the offending were "quite unusual".

He expressed surprise that the woman — who is also 19 — had received a three-month suspended sentence, after also being charged with common assault as a result of the altercation.

The court heard the photographs of both the man and woman after the altercation reflected an "equality of injury".

"I was somewhat taken aback by the sentence imposed upon her by a magistrate, namely a term of imprisonment of three months immediately suspended upon her entering into a good-behaviour order," Justice Elkaim said.

"I do not know what evidence was before the magistrate and it would be entirely inappropriate for me to criticise the decision.

"It does raise a difficulty for me in sentencing this offender because of the natural grievance [the woman] would feel if this offender received a lesser punishment for a more serious offence."

Despite those comments, Justice Elkaim said he would not allow the woman's sentence to dictate the offender's punishment.

Justice Elkaim said while a person who commits an offence of sexual intercourse without consent will "almost inevitably" go to prison, there were a series of "unusual facts" in this case.

He listed the offender's age, lack of criminal record, remorse and efforts he had taken to ensure he will not reoffend — such as attending courses — as mitigating factors.

He noted another "unusual element" in the case — that the offender and the woman still "enjoy a supportive, positive relationship".

Justice Elkaim ordered the man to sign an undertaking to comply with good behaviour obligations for two years, on the condition he attend relevant educational and psychological programs.

The man was ordered to sign a second undertaking for the common assault charge.