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. One who uses sex life not for sensual pleasures but only to beget children according to the reformatory method is also accepted as a brahmacari. One should not waste semen on sensual pleasure, violating the principles of Vedic life. Even if one is born in a family of dvijas, or twiceborn, if they have not followed the reformatory process he is called a dvija-bandhu–not one of the twiceborn, but a friend of the twice-born. The whole purpose of this system is to create good population. As stated in Bhagavad-gita, when women are polluted the populace is varna-sankara, and when the varnasankara population increases, the situation of the entire world becomes hellish. Therefore, all the Vedic literatures strongly warn against creating varna-sankara population. When there is varna-sankara population, the people cannot be properly controlled for peace and prosperity, regardless of great legislative assemblies, parliaments and similar bodies.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Seventh Canto, Chapter 11 – Text 13