So a sannyāsī is forbidden not to talk even in private place with woman. But a householder, he, if he associates woman under marriage tie, then it is religious. And without this, this is irreligious. And that religious sex life is God. Religious sex life is God. This should be followed. If we, every one of us reading Bhagavad-gītā, every one of us, at least... So far India is concerned, that is a different thing. In America also, I find so many American gentlemen, they read Bhagavad-gītā. But I am afraid if they are reading Bhagavad-gītā so scrutinizingly, as it is stated here, dharmāviruddho bhūteṣu kāmo 'smi bharatarṣabha: "Sex life which is not against religious principle, that is I am." So in, I mean to say, regulated sex life, married life, that is Kṛṣṇa. So that is not without Kṛṣṇa. That is Kṛṣṇa.

Then He says,

ye caiva sāttvikā bhāvā
rājasās tāmasāś ca ye
matta eveti tān viddhi
na tv ahaṁ teṣu te mayi
Now, one may question, "Then if You are particularly—You are Personality of Godhead, Supreme God—You are speaking that You are sound, You are this light, You are this praṇava, You are bīja, You are also kāma, so many things. So do You mean to say that You are simply in goodness? And what about the other things?" Because there are three qualities of the material world: the modes of goodness and the modes of passion and the modes of darkness. Now, so far Kṛṣṇa has described Himself, that any good thing... Just like sex life in marriage is a good. "That's all right. You are. What about other things?" Then Kṛṣṇa replies. He automatically says that ye caiva sāttvikā bhāvā rājasās tāmasāś ca ye.

(Lecture on Bhagavad-Gita------New York, 1966).

These instructions given by Lord Kṛṣṇa while talking with His friend the learned brāhmaṇa are very good for the guidance of human society. A system of human civilization that does not promote varṇa and āśrama is nothing but a polished animal society. Indulgence in sex life by a man or woman living single is never acceptable in human society. A man should strictly follow the principles of brahmacārī life or, with the permission of the spiritual master, should get married. Single life with illicit sex is animal life, for the animals have no such institution as marriage.

(Krsna Book).

So according to Vedic civilization, this training was given, student life, complete abstinence from sex life, then vānaprastha life, complete abstinence, and sannyāsa life, complete abstinence. The whole training was to abstain, to cure. Because... The same example: In diseased condition we cannot enjoy the foodstuff which we take. When we are healthy, we can enjoy the taste of the foodstuff. So we have to cure. We have to cure. And how to cure? To be situated in the transcendental position of Kṛṣṇa consciousness. That is the cure. So Kṛṣṇa advises here anyone who is able to tolerate the urge of sense pleasure. But we have to mold our life in such a way that we should be able to tolerate. Tolerate. That will give us our advancement in spiritual life, and when we are situated in spiritual life, that enjoyment is unending, unlimited. There is no end.

(Lecture on Bhagavad-Gita------New York, 1966).

The conditions for having good progeny in society are that the husband should be disciplined in religious and regulative principles and the wife should be faithful to the husband. In Bhagavad-gītā (7.11) it is said that sexual intercourse according to religious principles is a representation of Kṛṣṇa consciousness. Before engaging in sexual intercourse, both the husband and the wife must consider their mental condition, the particular time, the husband's direction, and obedience to the demigods. According to Vedic society, there is a suitable auspicious time for sex life, which is called the time for garbhādhāna. Diti neglected all the principles of scriptural injunction, and therefore, although she was very anxious for auspicious children, she was informed that her children would not be worthy to be the sons of a brāhmaṇa. There is a clear indication herein that a brāhmaṇa's son is not always a brāhmaṇa. Personalities like Rāvaṇa and Hiraṇyakaśipu were actually born of brāhmaṇas, but they were not accepted as brāhmaṇas because their fathers did not follow the regulative principles for their birth. Such children are called demons, or Rākṣasas. There were only one or two Rākṣasas in the previous ages due to negligence of the disciplinary methods, but during the age of Kali there is no discipline in sex life. How, then, can one expect good children? Certainly unwanted children cannot be a source of happiness in society, but through the Kṛṣṇa consciousness movement they can be raised to the human standard by chanting the holy name of God. That is the unique contribution of Lord Caitanya to human society.

(Srimad Bhagavatam------3:14:38------purport).