"Sex life, intoxication and meat eating are general tendencies of human society, but a regulated householder does not indulge in unrestricted sex life and other sense gratification. Marriage on principles of religious life is therefore current in all civilized human society because that is the way for restricted sex life. This restricted, unattached sex life is also a kind of yajna because the restricted householder sacrifices his general tendency toward sense gratification for higher transcendental life."
Bhagavad-gita 4:26 Purport
"As long as one is situated in the material world, there must be pleasure and pain arising from the material body. As Krsna advises in Bhagavad-gita, tams titiksasva bharata. One has to learn how to tolerate the temporary pains and pleasures of this material world. One must also be detached from his family and practice celibacy. Sex with one's wife according to the scriptural injunctions is also accepted as brahmacarya (celibacy), but illicit sex is opposed to religious principles, and it hampers advancement in spiritual consciousness.
"In Bhagavad-gita (7.11) it is said: dharmaviruddho bhutesu kamo 'smi bharatarsabha. Sex is allowed only for the begetting of children, not for enjoyment. One can indulge in sex to beget a good child for the benefit of the family, society and world. Otherwise, sex is against the rules and regulations of religious life. A materialistic person does not believe that everything is managed in nature, and he does not know that if one does something wrong, he is witnessed by different demigods. A person enjoys illicit sex, and due to his blind, lusty desire. he thinks that no one can see him, but this illicit sex is thoroughly observed by the agents of the Supreme Personality of Godhead. Therefore the person is punished in so many ways. presently in Kali-yuga there are many pregnancies due to illicit sex, and sometimes abortions ensue. These sinful activities are witnessed by the agents of the Supreme personality of Godhead, and a man and woman who create such a situation are punished in the future by the stringent laws of material nature (daivi hy esa guna-mayi mama maya duratyaya). Illicit sex is never excused, and those who indulge in it are punished life after life. As confirmed in Bhagavad-gita (16.20):
asurim yonim apanna
mudha janmani janmani
mam aprapyaiva kaunteya
tato yanty adhamam gatim
"Attaining repeated birth among the species of demoniac life, such persons can never approach Me. Gradually they sink down to the most abominable type of existence."
The Supreme Personality of Godhead does not allow anyone to act against the stringent laws of material nature; therefore illicit sex is punished life after life. Illicit sex creates pregnancies, and these unwanted pregnancies lead to abortion. Those involved become implicated in these sins, so much so that they are punished in the same way the next life. Thus in the next life they also enter the womb of a mother and are killed in the same way. All these things can be avoided by remaining on the transcendental platform of Krsna consciousness. In this way one does not commit sinful activity. Illicit sex is the most prominent sin due to lusty desire. When one associates with the mode of passion, he is implicated in suffering life after life.
Srimad-Bhagavatam 5:14:9 Purport
"The natural instinct of a woman is to enjoy the material world. She induces her husband to enjoy this world by satisfying his tongue, belly and genitals, which are called jihva, udara and upastha. A woman is expert in cooking palatable dishes so that she can easily satisfy her husband in eating. When one eats nicely, his belly is satisfied, and as soon as the belly is satisfied the genitals become strong. Especially when a man is accustomed to eating meat and drinking wine and similar passionate things, he certainly becomes sexually inclined. It should be understood that sexual inclinations are meant not for spiritual progress but for gliding down to hell. Thus Kasyapa Muni considered his situation and lamented. In other words, to be a householder is very risky unless one is trained and the wife is a follower of her husband. A husband should be trained at the very beginning of his life. Kaumara acaret prajno dharman bhagavatan iha (Bhag. 7.6.1). During the time of brahmacarya, or student life, a brahmacari should be taught to be expert in bhagavata-dharma, devotional service. Then when he marries, if his wife is faithful to her husband and follows him in such life, the relationship between husband and wife is very desirable. However, a relationship between husband and wife without spiritual consciousness but strictly for sense gratification is not at all good. It is said in Srimad-Bhagavatam (12.2.3) that especially in this age, Kali-yuga, dam-patye 'bhirucir hetuh: the relationship between husband and wife will be based on sexual power. Therefore householder life in this Kali-yuga is extremely dangerous unless both the wife and husband take to Krsna consciousness."
Srimad-Bhagavatam 6:18:40 Purport
"Material life is such that due to indulgence in illicit sex, gambling, intoxication and meat-eating, the conditioned soul is always in a dangerous condition. Meat-eating and intoxication excite the senses more and more, and the conditioned soul falls victim to women. In order to keep women, money is required, and to acquire money, one begs, borrows or steals. Indeed, he commits abominable acts that cause him to suffer both in this life and in the next. Consequently illicit sex must be stopped by those who are spiritually inclined or who are on the path of spiritual realization. Many devotees fall down due to illicit sex. They may steal money and even fall down from the highly honored renounced order. Then for a livelihood they accept menial services and become beggars. It is therefore said in the sastras, yan maithunadi-grhamedhi-sukham hi tuccham: materialism is based on sex, whether licit or illicit. Sex is full of dangers even for those who are addicted to household life. Whether one has a license for sex or not, there is great trouble. Bahu-duhkha-bhak: after one indulges in sex, many volumes of miseries ensue. One suffers more and more in material life. A miserly person cannot properly utilize the wealth he has, and similarly a materialistic person misuses the human form. Instead of using it for spiritual emancipation, he uses the body for sense gratification. Therefore he is called a miser."
Srimad-Bhagavatam 5:14:22 Purport
"Materialistic life is based on sex life. The existence of all the materialistic people, who are undergoing severe tribulation in the struggle for existence, is based on sex. Therefore, in the Vedic civilization sex life is allowed only in a restricted way; it is for the married couple and only for begetting children. But when sex life is indulged in for sense gratification illegally and illicitly, both the man and the woman await severe punishment in this world or after death. In this world also they are punished by virulent diseases like syphilis and gonorrhea, and in the next life, as we see in this passage of Srimad-Bhagavatam, they are put into different kinds of hellish conditions to suffer."
Srimad-Bhagavatam 3:30:28 Purport
"Generally a man should not have sexual relations with any woman other than his wife. According to Vedic principles, the wife of another man is considered one's mother, and sexual relations are strictly forbidden with one's mother, sister and daughter. If one indulges in illicit sexual relations with another man's wife, that activity is considered identical with having sex with one's mother. This act is most sinful. The same principle holds for a woman also; if she enjoys sex with a man other than her husband, the act is tantamount to having sexual relations with her father or son. Illicit sex life is always forbidden, and any man or woman who indulges in it is punished in the manner described in this verse."
Srimad-Bhagavatam 5:26:20 Purport
"Those who strongly desire to cross the ocean of nescience must not associate with the modes of ignorance, for hedonistic activities are the greatest obstructions to realization of religious principles, economic development, regulated sense gratification and, at last, liberation.
Purport: The four principles of life allow one to live according to religious principles, to earn money according to one's position in society, to allow the senses to enjoy the sense objects according to regulations, and to progress along the path of liberation from this material attachment. As long as the body is there, it is not possible to become completely free from all these material interests. It is not, however, recommended that one act only for sense gratification and earn money for that purpose only, sacrificing all religious principles. At the present moment, human civilization does not care for religious principles. It is, however, greatly interested in economic development without religious principles. For instance, in a slaughterhouse the butchers certainly get money easily, but such business is not based on religious principles. Similarly, there are many nightclubs for sense gratification and brothels for sex. Sex, of course, is allowed in married life, but prostitution is prohibited because all our activities are ultimately aimed at liberation, at freedom from the clutches of material existence. Similarly, although the government may license liquor shops, this does not mean that liquor shops should be opened unrestrictedly and illicit liquor smuggled. Licensing is meant for restricting. No one has to take a license for sugar, wheat or milk because there is no need to restrict these things. In others words, it is advised that one not act in a way that will obstruct the regular process of advancement in spiritual life and liberation. The Vedic process of sense gratification is therefore planned in such a way that one can economically develop and enjoy sense gratification and yet ultimately attain liberation. Vedic civilization offers us all knowledge in the sastras, and if we live a regulated life under the direction of sastras and guru, all our material desires will be fulfilled; at the same time we will be able to go forward to liberation."
Srimad-Bhagavatam 4:22:34 Purport
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.