Illicit sex is the most prominent sin due to lusty desire.
“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 Krishna 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. ”
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 14 – Text 09
