In the rules of celibacy written by the great sage Yājñavalkya it is said:
karmaṇā manasā vācā
sarvāvasthāsu sarvadā
sarvatra maithuna-tyāgo
brahmacaryaṁ pracakṣate
"The vow of brahmacarya is meant to help one completely abstain from sex indulgence in work, words and mind-at all times, under all circumstances, and in all places." No one can perform correct yoga practice through sex indulgence. Brahmacarya is taught, therefore, from childhood, when one has no knowledge of sex life. Children at the age of five are sent to the guru-kula, or the place of the spiritual master, and the master trains the young boys in the strict discipline of becoming brahmacārīs. Without such practice, no one can make advancement in any yoga, whether it be dhyāna, jñāna or bhakti.

(Bhagavad-Gita-------6:13-14--------purport).

When one becomes situated in the transcendental position by various philosophical researches, he is said to have achieved samprajñāta-samādhi. In the asamprajñāta-samādhi there is no longer any connection with mundane pleasure, for one is then transcendental to all sorts of happiness derived from the senses. When the yogī is once situated in that transcendental position, he is never shaken from it. Unless the yogī is able to reach this position, he is unsuccessful. Today's so-called yoga practice, which involves various sense pleasures, is contradictory. A yogī indulging in sex and intoxication is a mockery. Even those yogīs who are attracted by the siddhis (perfections) in the process of yoga are not perfectly situated. If yogīs are attracted by the by-products of yoga, then they cannot attain the stage of perfection, as is stated in this verse. Persons, therefore, indulging in the make-show practice of gymnastic feats or siddhis should know that the aim of yoga is lost in that way.

(Bhagavad-Gita-------6:20:23--------purport).

The varṇāśrama-dharma is prescribed for the civilized human being just to train him to successfully terminate human life. Self-realization is distinguished from the life of the lower animals engaged in eating, sleeping, fearing and mating. Bhīṣmadeva advised for all human beings nine qualifications: (1) not to become angry, (2) not to lie, (3) to equally distribute wealth, (4) to forgive, (5) to beget children only by one's legitimate wife, (6) to be pure in mind and hygienic in body, (7) not to be inimical toward anyone, (8) to be simple, and (9) to support servants or subordinates. One cannot be called a civilized person without acquiring the above-mentioned preliminary qualities. Besides these, the brāhmaṇas (the intelligent men), the administrative men, the mercantile community and the laborer class must acquire special qualities in terms of occupational duties mentioned in all the Vedic scriptures. For the intelligent men, controlling the senses is the most essential qualification. It is the basis of morality. Sex indulgence even with a legitimate wife must also be controlled, and thereby family control will automatically follow. An intelligent man abuses his great qualifications if he does not follow the Vedic way of life. This means he must seriously make a study of the Vedic literatures, especially of the Śrīmad-Bhāgavatam and the Bhagavad-gītā. For learning Vedic knowledge, one must approach a person who is cent percent engaged in devotional service. He must not do things which are forbidden in the śāstras. A person cannot be a teacher if he drinks or smokes. In the modern system of education the teacher's academic qualification is taken into consideration without evaluation of his moral life. Therefore, the result of education is misuse of high intelligence in so many ways.

(Srimad Bhagavatam--------1:9:26--------purport).

The lifetime of such an envious householder is passed at night either in sleeping or in sex indulgence, and in the daytime either in making money or maintaining family members.
The present human civilization is primarily based on the principles of sleeping and sex indulgence at night and earning money in the day and spending the same for family maintenance. Such a form of human civilization is condemned by the Bhāgavata school.

Because human life is a combination of matter and spirit soul, the whole process of Vedic knowledge is directed at liberating the spirit soul from the contamination of matter. The knowledge concerning this is called ātma-tattva. Those men who are too materialistic are unaware of this knowledge and are more inclined to economic development for material enjoyment. Such materialistic men are called karmīs, or fruitive laborers, and they are allowed regulated economic development or association of woman for sex indulgence. Those who are above the karmīs, that is, the jñānīs, yogīs and devotees, are strictly prohibited from sex indulgence. The karmīs are more or less devoid of ātma-tattva knowledge, and as such, their life is spent without spiritual profit. The human life is not meant for hard labor for economic development, nor is it meant for sex indulgence like that of the dogs and hogs. It is specially meant for making a solution to the problems of material life and the miseries thereof. So the karmīs waste their valuable human life by sleeping and sex indulgence at night, and by laboring hard in the daytime to accumulate wealth, and after doing so, they try to improve the standard of materialistic life. The materialistic way of life is described herein in a nutshell, and how foolishly men waste the boon of human life is described as follows.

(Srimad Bhagavatam--------2:1:3--------translation and purport).

The illusory energy of matter is so strong that one is apt to be under such illusion at every stage of life, even after quitting one's happy home. Therefore, it is essential that one practice self-control by celibacy without the least desire for sex indulgence. For a man desiring to improve the condition of his existence, sex indulgence is considered suicidal, or even worse. Therefore, to live apart from family life means to become self-controlled in regard to all sense desires, especially sex desires. The method is that one should have a duly sanctified sitting place made of straw, deerskin and carpet, and thus sitting on it one should chant the holy name of the Lord without offense, as prescribed above. The whole process is to drag the mind from material engagements and fix it on the lotus feet of the Lord. This simple process alone will help one advance to the highest stage of spiritual success.

(Srimad Bhagavatam--------2:1:16--------purport).

So purification means getting free gradually from sex desire, and this is attained by meditation on the person of the Lord as described herein, beginning from the feet. One should not try to go upwards artificially without seeing for himself how much he has been released from the sex desire. The smiling face of the Lord is the Tenth Canto of Śrīmad-Bhāgavatam, and there are many upstarts who at once try to begin with the Tenth Canto and especially with the five chapters which delineate the rāsa-līlā of the Lord. This is certainly improper. By such improper study or hearing of Bhāgavatam, the material opportunists have played havoc by indulgence in sex life in the name of Bhāgavatam. This vilification of Bhāgavatam is rendered by the acts of the so-called devotees; one should be free from all kinds of sex desire before he tries to make a show of recital of Bhāgavatam. Śrī Viśvanātha Cakravartī Ṭhākura clearly defines the import of purification as cessation from sex indulgence. He says, yathā yathā dhīś ca śudhyati viṣaya-lāmpaṭyaṁ tyajati, tathā tathā dhārayed iti citta-śuddhi-tāratamyenaiva dhyāna-tāratamyam uktam. And as one gets free from the intoxication of sex indulgence by purification of intelligence, one should step forward for the next meditation, or in other words, the progress of meditation on the different limbs of the transcendental body of the Lord should be enhanced in proportion to the progress of purification of the heart. The conclusion is that those who are still entrapped by sex indulgence should never progress to meditation above the feet of the Lord; therefore recital of Śrīmad-Bhāgavatam by them should be restricted to the First and Second Cantos of the great literature. One must complete the purificatory process by assimilating the contents of the first nine cantos. Then one should be admitted into the realm of the Tenth Canto of Śrīmad-Bhāgavatam.

(Srimad Bhagavatam--------2:2:12--------purport).

As a fire can consume anything and everything without being contaminated, so, by the grace of the Lord, the fire of Brahmā's greatness consumed his desire for the sinful act of sex with his daughter. The Vedas are the source of all knowledge, and they were first revealed to Brahmā by the mercy of the Supreme Personality of Godhead while Brahmā was thinking of re-creating the material world. Brahmā is powerful by dint of his devotional service unto the Lord, and the Lord is always ready to forgive His devotee if by chance he falls down from the noble path of devotional service. The Śrīmad-Bhāgavatam (11.5.42) confirms this as follows:
sva-pāda-mūlaṁ bhajataḥ priyasya
tyaktvānya-bhāvasya hariḥ pareśaḥ
vikarma yac cotpatitaṁ kathañ-cid
dhunoti sarvaṁ hṛdi sannviṣṭaḥ
"Any person who is engaged one hundred percent in the transcendental loving service of the Lord, at His lotus feet, is very dear to the Personality of Godhead Hari, and the Lord, being situated in the heart of the devotee, excuses all kinds of sins committed by chance." It was never expected that a great personality like Brahmā would ever think of sex indulgence with his daughter. The example shown by Brahmā only suggests that the power of material nature is so strong that it can act upon everyone, even Brahmā. Brahmā was saved by the mercy of the Lord with a little punishment, but by the grace of the Lord he did not lose his prestige as the great Brahmā.

(Srimad Bhagavatam--------3:12:34--------purport).