Chris Kates, general secretary of the National Association of Schoolmasters Union of Women Teachers (NASUWT), is agitated that teachers having sex with their pupils aged above 16 should go on the sex offenders register.
Those entering the register would have their movements strictly monitored by the police. Any infraction could lead them to jail.
Kates' contention is it is unfair to look upon such relationships as a crime even where they start after the pupil reaches the age of consent.
Kates told ITV's Tonight programme the law was wrong because a teacher could legally enter a relationship with a sixth former at another school.
Child protection professionals have criticised her comments.
Since 1991, 129 teachers have been prosecuted for relationships with pupils, but a Sheffield University study suggested as many as 1,500 intimate relationships develop every year.
In 2001, the law was changed to make it illegal for teachers to engage in sexual activity with pupils at their school when the student is aged under 18.
Kates said: "If a teacher has a relationship with a pupil at the school at which they teach, it could be an 18-year-old pupil in sixth form, then that teacher can still be prosecuted and end up on the sex offenders register.
"Clearly there have to be appropriate disciplinary sanctions in the school where a teacher works to make sure that inappropriate relationships don't develop.
"But it does seem a step too far, when there has been a consensual relationship, to put that person on the sex offenders register when, in fact, they could have a perfectly legitimate relationship with an 18-year-old at another school."
Later, Keates told the BBC the NASUWT had raised the anomaly as soon as the legislation was drafted but also insisted that it advises members to keep relationships professional at all times.
Any teacher or, for that matter, any adult employee found sexually abusing pupil of any age should face the full legal consequences, she stressed.
NSPCC policy advisor Zoe Hilton said: "The law is very clear that if a teacher abuses his or her position by forming a sexual relationship with a pupil they could be prosecuted and this remains the case even if the child gives their consent.
"The law is, quite rightly, there to protect children."