The mind, like the flame, is susceptible to so many material desires that with the slightest agitation it will move. A little movement of the mind can change the whole consciousness. Therefore in India one seriously practicing yoga traditionally remained brahmacari, or celibate. There are two kinds of brahmacari: one is completely celibate and the other is grhastha-brahmacari, that is to say he has a wife, he does not associate with any other woman, and his relations with his own wife are strictly regulated. In this way, either by complete celibacy or restricted sex life, one's mind is kept from being agitated. Yet when one takes a vow to remain a complete celibate, his mind may still be agitated by sexual desire; therefore in India those practicing the traditional yoga under strict vows of celibacy are not allowed to sit alone even with a mother, sister or daughter. The mind is so fickle that the slightest suggestion can create havoc.

(From: The Perfection of Yoga)