SB 4.25.38

Since there are 8,400,000 species of life, there are also many different living conditions. In the lower grades of life (in plant and tree life) there is no system for sexual intercourse. In the upper grades (in the life of birds and bees) there is sex, but the insects and animals do not know how to actually enjoy sex life. In the human form of life, however, there is full knowledge of how to enjoy sex. Indeed, there are many so-called philosophers who give directions on how to enjoy sex life. There is even a science called kāma-śāstra, which is the science of sex. In human life there are also such divisions as brahmacarya, gṛhastha, vānaprastha and sannyāsa. There is no sex life except in the gṛhastha, or householder, āśrama. The brahmacārī is not allowed any sex, a vānaprastha voluntarily refrains from sex, and the sannyāsī is completely renounced. The karmīs do not practice brahmacarya, vānaprastha or sannyāsa life, for they are very much interested in gṛhastha life. In other words, a human being is very much materially inclined. Indeed, all living entities are materially inclined. They prefer gṛhastha life because there is a concession for sex. The karmīs think the other statuses of life are worse than animal life, for animals also have sex whereas the brahmacārī, vānaprastha and sannyāsī completely give up sex. The karmīs, therefore, abhor these orders of spiritual life.