Marriage and family serve as the foundation on which our society takes its shape and develops and are thus institutionally protected by the Constitution (See Interpretations Nos. 362 and 552). The root of our marriage system lies in the freedom of personality, with such social functions as the maintenance of the order of human relationships and gender equality, and the raising of children. To insure an enduring and unimpaired system of marriage, the state may of course enact relevant rules to require the husband and the wife to be mutually bound to each other by the duty of faithfulness. The freedom of sexual behavior is inseparably related with the personality of individuals, and every person is free to decide whether or not and with whom to have sexual affairs. Such freedom is, however, legally protected only if it is not detrimental to the social order or public interest as it is so provided in Article 22 of the Constitution. Thus, the freedom of sexual behavior is subject to the restriction put on it by marriage and the family system.　　

　　What type of restriction, if any, must be imposed on sexual affairs between a married person and a third party during the subsistence of a marriage and whether or not an act in violation of such restriction should be made punishable as a crime are problems that must be weighed and determined by the legislature by taking into consideration the customs of the country, which vary between nations. While the Criminal Code, by providing in Article 239 that a person who commits adultery and the other party to the adultery are punishable with imprisonment for not more than one year, imposes a restriction on the freedom of sexual behavior, such restriction is essential in order to safeguard marriage, the family system, and the social order. To avoid overly severe restrictions, however, the Code establishes certain ancillary conditions on the prosecution of the offense of adultery by setting forth in Article 245, Paragraph 1, that the offense is indictable only upon complaint, and in Paragraph 2 of the same Article that no complaint may be instituted if the spouse has connived at or forgiven his wife or her husband for the offense. These statutes are reflective of the value judgment made by the lawmakers to balance the preservation of the systems of marriage and family against the freedom of sexual behavior, and do not go beyond the sphere of discretion of the legislative power nor are they in conflict with the principle of proportionality embodied in Article 23 of the Constitution.

　　Marriage and family serve as the foundation on which our society takes its shape and develops and are thus institutionally protected by the Constitution (See Interpretations Nos. 362 and 552). The root of our marriage system lies in the freedom of personality, with such social functions as the maintenance of the order of human relationships and gender equality, and the raising of children. To insure an enduring and unimpaired system of marriage, the state may of course enact rules to require the husband and the wife to be mutually bound to each other by the duty of faithfulness. The freedom of sexual behavior is inseparably related with the personality of individuals, and every person is free to decide whether or not and with whom to have sexual affairs. Such freedom is, however, legally protected only if it is not detrimental to the social order or public interest as it is so provided in Article 22 of the Constitution. Thus, the freedom of sexual behavior is subject to the restriction put on it by marriage and the family system.

　　Marriage means a living agreement where a husband and a wife mutually engage with each other to live their lives together so that both may realize and develop their respective personalities. The relation of a lifetime association formed by marriage not only unites the husband and the wife in a relationship of mutual support and reliance in spirit and material but also is enlarged to serve as a foundation for families and the society. As regards the type of restriction, if any, that must be imposed on sexual affairs between a married person and a third party during the subsistence of a marriage and whether or not an act in violation of such restriction should be made punishable as a crime, the problems must be dealt with by the norms of conduct to be determined by the legislature by taking into consideration how marriages and the family system should be protected in light of the customs of the country, which vary from nation to nation. If the legislature chooses to make such an act punishable under criminal law and considers that criminal punishment will be helpful to the achievement of justifiable legislative purposes, and that there is available no other alternative means to attain the same purposes with less harm, the law should not be deemed to be inconsistent with the principle of proportionality embodied in Article 23 of the Constitution insofar as the restriction imposed by criminal punishment on the fundamental right is proportional to the importance of the legal interest intended by the lawmakers to be placed under protection and the degree of detriment to be caused by the act to the legal interest.

　　The cornerstone supporting matrimonial cohabitation is unquestionably the relationship between the husband and wife established upon the affection and faithfulness toward each other. With no better alternative means, the law adopts a criminal punishment approach to the enforcement of a restriction on the freedom of sexual behavior between married persons and others by providing in Article 239 of the Criminal Code that "a married person who commits adultery with another shall be punished with imprisonment for not more than one year; the other party to the adultery shall be liable to the same punishment." In a sense, the general preventive function of the criminal law is particularly effective in making the husband and wife adhere to their duty of loyalty and faithfulness to one another so that such duty may become the fundamental norm of social life and may, furthermore, promote the people's legal consciousness of regarding marriage highly and safeguarding the ethical value of marriage and the family system. To the extent that the judgment made by the legislature on the duty of loyalty and faithfulness between husband and wife is not contrary to the general concept of the general populace and that the people's observance of such norms of duty is not beyond expectation, the legislature may certainly resort to criminal punishment for the legislative purposes of preventing the offense of adultery and protecting marital relations. Besides, the penalty of imprisonment for one year or less for adultery under the Criminal Code makes the act a minor offense under Article 61 thereof; Paragraph 1 of Article 245 of the Code makes the offense of adultery indictable only upon complaint, thus giving the injured spouse the opportunity to take into consideration the affection between the husband and the wife and their privacy so as to avoid the possibility of family breakup in consequence of the institution of a complaint on the ground of adultery; and under Paragraph 2 of the same Article the spouse is debarred from filing a complaint if he or she has connived at or has forgiven the other party for the act. These statutes have set a limit on punishment for adultery to an essential degree by requiring additional prerequisites for prosecution of the offense, and are thus not in conflict with Article 23 of the Constitution.