New laws forbidding sexual relationships between young people and adults in authority are fundamentally flawed, says a peak legal body. But the ACT government is unrepentant, saying the laws reflect the expectations of the community.

The new act makes it a criminal offence for an adult with responsibility for a young person to have a sexual relationship with that young person, even if they consent. The list of position-of-authority relationships includes teachers, foster or step-parents, counsellors, health professionals, youth workers and sports coaches.

Those suspected of flouting the laws can now be charged with the two new offences of sexual intercourse and act of indecency with a 16- or 17-year-old.

The laws passed in the ACT Legislative Assembly on Tuesday despite vocal opposition from many in the territory's legal fraternity. In a letter to ACT Attorney-General Simon Corbell last month, the ACT Law Society warned the law did not acknowledge pre-existing relationships.

''An example could be when a 21-year-old person is engaged in a sexual relationship with a 17-year-old and then after that relationship commences, the 21-year-old becomes the coach of their 17-year-old partner's sporting team,'' the letter said.

''This should not create an offence because what would in effect be being criminalised is the coaching activity rather than the sex. The offence should require that the special relationship in some way procured the sex and/or vitiated the young person's consent.''

The letter also said the act granted marriage a privileged status above de facto relationships, which would discriminate against same-sex couples.

In November last year, defence lawyer Ben Aulich, principal of Ben Aulich & Associates, in a letter to Fairfax Media described the move as ''another knee-jerk reaction''. Mr Aulich said the laws could lead to otherwise law-abiding citizens being placed on the sex offender register and serving 10 years behind bars.

''Like it or not, we all know young people today are having sex much earlier than they ever have,'' he said. ''The broad definition of 'special care' is very worrying.''

But Mr Corbell said the views of the ACT Law Society and other groups were taken into account during development of the new offences. He said the provisions were about protecting young people from the sexual advances of people in positions of authority who should be protecting them.

''The new offences recognise that in these relationships, a 16- or 17-year-old cannot give effective consent to sexual activity,'' Mr Corbell said.

''Once you are in a special relationship with a young person, you are on notice that you should not be having sex with them.

''The overwhelming majority of the community, including those caring for young people, agree this is the case.''