In the varnasrama system, the first ceremony for purification is garbhadhana, which is performed with mantras at the time of sex for propagating a good child
SB Canto 7
Those who have been reformed by the garbhādhāna ceremony and other prescribed reformatory methods, performed with Vedic mantras and without interruption, and who have been approved by Lord Brahmā, are dvijas, or twice-born. Such brāhmaṇas, kṣatriyas and vaiśyas, purified by their family traditions and by their behavior, should worship the Lord, study the Vedas and give charity. In this system, they should follow the principles of the four āśramas (brahmacarya, gṛhastha, vānaprastha and sannyāsa).
After giving a general list of thirty qualifications for one's behavior, Nārada Muni now describes the principles of the four varṇas and four āśramas. A human being must be trained in the above-mentioned thirty qualities; otherwise, he is not even a human being. Then, among such qualified persons, the varṇāśrama process should be introduced. In the varṇāśrama system, the first ceremony for purification is garbhādhāna, which is performed with mantras at the time of sex for propagating a good child. One who uses sex life not for sensual pleasures but only to beget children according to the reformatory method is also accepted as a brahmacārī. One should not waste semen on sensual pleasure, violating the principles of Vedic life. Restraint in sex is possible, however, only when the populace is trained in the above-mentioned thirty qualities; otherwise, it is not possible. Even if one is born in a family of dvijas, or twice-born, if they have not followed the reformatory process he is called a dvija-bandhu—not one of the twice-born, but a friend of the twice-born. The whole purpose of this system is to create good population. As stated in Bhagavad-gītā, when women are polluted the populace is varṇa-saṅkara, and when the varṇa-saṅkara population increases, the situation of the entire world becomes hellish. Therefore, all the Vedic literatures strongly warn against creating varṇa-saṅkara population. When there is varṇa-saṅkara population, the people cannot be properly controlled for peace and prosperity, regardless of great legislative assemblies, parliaments and similar bodies.