There are eight kinds of subtle sex life. If you see one beautiful woman and if you appreciate, "Oh, how nice the face is," that is subtle sex. If you read books, that is also subtle sex. If you endeavor how to approach that woman or man to find out the opportunity, that is subtle sex. There are eight kinds of subtle sex life. So it is forbidden for a brahmacari even to think of woman. That is brahmacari. Even thinking of woman is subtle sex life. It is very, very difficult. But mam eva ye prapadyante mayam etam taranti te [Bg. 7.14]. If you catch the lotus feet of Krsna very tightly, these things will not disturb. That is said by Yamunacarya. Bhavati mukha-vikarah. If you become pure devotee of Krsna, then as soon as you think of sex, you'll spit: "Eh! Nonsense. What is this?" This is the result. That is the certificate, whether you have got that attitude — spit on it. Then you understand that you are increasing. Otherwise you are in the same hellish condition. You may show that you are becoming a devotee, very great devotee, so test yourself how much you have advanced.

From Srila Prabhupada's lecture on Srimad-Bhagavatam 7.6.4 — December 5, 1975, Vrndavana