Because teachers are in a position of authority over their students. The law doesn't want a teacher having a sexual relationship, or any romantic relationship at all, with a student. ... Teachers can have an ability to influence a studentin ways that the student might not have considered before entering the relationship
The law says that anyone in a position of trust cannot have any sexual contact with a person in their care, even when they are over the age of consent at 16. The reason for this is that the adult is in a position of responsibility and they are there to look out for the best interests of the young person in their care. Someone in a position of trust is classed as a teacher or someone whose job it is to look after you, so if you were in hospital, or in residential care, the people looking after you would also be breaking the law if they were to have a sexual relationship with you. This means that any sexual activity with someone under 18 is actually illegal, which includes any intimate touching, even kissing, as well as all sexual intercourse between both males and females.
It is important to remember that the person over the age of 18 is the one who is breaking the law, and the person receiving the care, such as the student, and are therefore deemed to be innocent. No matter what part they have played in the relationship, even if they want the relationship, and have taken active steps to encourage it where no charges will be brought against them.
Even when both people are over 18, many jobs where someone provides care or a service for someone else, have rules in place which make it against their policy to engage in a sexual relationship. If the worker ignores this rule, it can mean they can lose their job because the relationship is seen as inappropriate, and therefore goes against the company policies and procedures, which are there to protect the people who are being provided a service or being cared for.