Srimad Bhagavatam Canto 09, Chapter 19, Text 17

SB 9.19.17

matra svasra duhitra va
 naviviktasano bhavet
balavan indriya-gramo
 vidvamsam api karsati
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
One should not allow oneself to sit on the same seat even with one’s own mother, sister or daughter, for the senses are so strong that even though one is very advanced in knowledge, he may be attracted by sex.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Learning the etiquette of how to deal with women does not free one from sexual attraction. As specifically mentioned herewith, such attraction is possible even with one’s mother, sister or daughter. Generally, of course, one is not sexually attracted to his mother, sister or daughter, but if one allows himself to sit very close to such a woman, one may be attracted. This is a psychological fact. It may be said that one is liable to be attracted if he is not very advanced in civilized life; however, as specifically mentioned here, vidvamsam api karsati: even if one is highly advanced, materially or spiritually, he may be attracted by lusty desires. The object of attraction may even be one’s mother, sister or daughter. Therefore, one should be extremely careful in dealings with women. Sri Caitanya Mahaprabhu was most strict in such dealings, especially after He accepted the sannyasa order. Indeed, no woman could come near Him to offer Him respect. Again, one is warned herewith that one should be extremely careful in dealings with women. A brahmacari is forbidden even to see the wife of his spiritual master if she happens to be young. The wife of the spiritual master may sometimes take some service from the disciple of her husband, as she would from a son, but if the wife of the spiritual master is young, a brahmacari is forbidden to render service to her.
Srimad Bhagavatam Canto 09, Chapter 19, Text 16
Srimad Bhagavatam Canto 09, Chapter 19, Text 18