Having other fantasies or behaviour than the average person in
relation to sex is not punishable. The behaviour that can belong to these fantasies (for example sex with minors, showing your genitals to people who are bothered by this) can be punishable if others evidently suffer from this.
Penal Code makes a number of sexual behaviours punishable:
sexually penetrate an opening of the body by (threatening to use) violence or by other means -- for example psychological pressure -- of a man or a woman, within or outside of marriage (article 242).
force a person to commit or allow sexual acts other than penetration by (threatening to use) violence or by other means (article 246).
sexually penetrate a child younger than 12 years old, or a child between 12 and 16 years old, but in that case only when the child, a parent or Child Welfare files a complaint, or an unconscious person or a person with a serious physical, mental or psychological handicap. Also other sexual acts than penetrating a person of these categories are punishable. Sexual abuse with misuse of authority: sexual contact with someone younger than 18 years old in a dependence relation (parent-child, teacher-pupil) or with an adult who has been put under the care of the authority of the perpetrator (doctor-patient, social worker- client, teacher-pupil) (articles 240, 240a and 240b).
Act of indecency:
deliberately show the genitals (exhibitionism) or commit sexual acts in public (article 239).
offer unasked-for pornography, offer asked-for or unasked-for pornography to someone younger than 16 years old and use someone younger than 16 years old to make child pornography for distribution (articles 240, 240a and 240b)
The above is only a summary, without all special conditions.
have similar laws, but with variations. In many countries, not only production but even owning child pornography is forbidden. The exact age limit for sexual abuse of children may vary up and down a little between the above cited Dutch penal code. Homosexuality, sexual relations outside marriage and sexual relations with animals are examples of activities which in some countries and at some times has been forbidden.
Many countries also have incest laws making sexual relations between close relatives, such as parents and children, or between brothers and sisters, illegal, but exactly what is illegal varies between countries.
Many countries also outlaw the use of contraceptives and abortion. The reverse, forbidding more than one child, is specified in Chinese law.
is illegal in some countries, an extreme variant of this is some Muslim countries who do not even allow women to show their hair publicly.
is illegal in some countries, but in some countries the prostitute is sentenced, in other countries the customer of the prostitute, in other countries only persons who make money from other people's prostitution. In some countries, prostitution is legal, but regulated by special laws with the intention to protect the prostitutes and reduce the risk of diseases.
or extramarital sex is outlawed in many countries. Often, however, there is no punishment for infidelity.
Legal control of sexual behaviour was in historical time often stricter, and is even today much stricter in some non-European countries. Religions are also commonly used to control sexual behaviour in various ways.