Why would she believe such things? I have to speculate here, not being able to read the original documents in Norwegian, but I expect there is an intellectual lineage running back to psychological theorists like Wilhelm Reich.
Reich lived in Norway in the 1930s. Although some of his ideas aroused popular hostility there, forcing him to relocate to the US, his students carried on his work:
Reich was a substantial influence in Norwegian psychology and sexology ... Ola Raknes, one of Reich’s most prominent Norwegian proponents, did extensive research on the results of the socialization of sexuality in childhood. Raknes was able to identify many adult sexual problems that were related to the repression of the natural sexual urge in childhood.
This is how Wikipedia describes Reich's theories:
Reich agreed with Freud that sexual development was the origin of mental disorder. They both believed that most psychological states were dictated by unconscious processes; that infant sexuality develops early but is repressed, and that this has important consequences for mental health. At that time a Marxist, Reich argued that the source of sexual repression was bourgeois morality and the socio-economic structures that produced it. As sexual repression was the cause of the neuroses, the best cure would be to have an active, guilt-free sex life.
Our Norwegian kindergarten teacher is therefore a bit dated in her views: she is regurgitating theories which have been around for several generations. She is replaying a scene first from the 1930s and then from the 1970s:
Thore Langfeldt, a pioneer in the field of sexology in Norway, began ... his groundbreaking research with children. During the 1970s, he supervised several students, Jens Skaar, Bjørn Helge Gundersen, and Per Steinar Melås, in their task of collecting information from parents and nursery staff about the sexual behavior observed among children. The findings, published in Hverdag, caused a public protest against the invasion of the pristine world of children by lewd professionals.
How can you explain the Reichian view? You could try to explain it in terms of Reich's personal life. Reich's mother committed suicide after an affair with his tutor; his shattered father followed suit not long after. Reich had a particular reason, therefore, to focus on the effects of sexual guilt.
Why, though, would Reich's views take hold amongst other members of the intelligentsia? I expect a full answer would have to consider the following:
a) Reich was proposing an all-encompassing, unified, scientific explanation for the existence of problems in the individual and society. Modernists hunger for this kind of theory.
b) Reich's ideas fit well with liberal autonomy theory: the view that the overriding good is a self-determination in which there are no impediments to our will. Sexual morality limits what we can choose and therefore comes to be thought of negatively as a restriction on individual autonomy from which individuals are to be liberated.
c) If you are an intellectual in a state of rancour against your society and civilisation, subverting sexual mores might be thought of as an effective blow against the existing order.
As for the Norwegians, they need to get over the 1930s. Reich is generally thought of now as a mad scientist (he apparently claimed to have done battle with alien spaceships). There must be more fitting approaches to child psychology than the ideological one he represents.