One should not waste semen on sensual pleasure.

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