The prime thing that keeps one bound in the material world is the attraction for the opposite sex. This attraction overpowers all other attractions of this world.
puṁsaḥ striyā mithunī-bhāvam etaṁ
tayor mitho hṛdaya-granthim āhuḥ
ato gṛha-kṣetra-sutāpta-vittair
janasya moho ‘yam ahaṁ mameti
The attraction between male and female is the basic principle of material existence. On the basis of this misconception, which ties together the hearts of the male and female, one becomes attracted to his body, home, property, children, relatives and wealth. In this way one increases life’s illusions and thinks in terms of “I and mine.”
A renounced person has to be continuously on guard with the enemies that may lead him to ponder over sense gratificatory thoughts in the mind. What to speak of sense gratification, even thoughts concerning having any kind of mental dialogue with a woman, or simply thoughts of woman even with no negative feeling, that is also a mental falldown.
Shrila Prabhupada writes, “To see a woman’s face and appreciate its beauty or to hear a woman’s voice and appreciate her singing as very nice is a subtle falldown for a brahmacārī or sannyāsī.” SB 6.18.41 P
After the incident of Mahaprabhu banishing Chota Haridas, it is said in Caitanya Caritamrita Antya Lila 2.144,
dekhi’ trāsa upajila saba bhakta-gaṇe
svapne-ha chāḍila sabe strī-sambhāṣaṇe
After all the devotees saw this example, a mentality of fear grew among them. Therefore they all stopped talking with women, even in dreams.
This doesn’t only refer to night dreams, but even day dreams!! Dreaming with eyes open! Members in renounced order must not replay, re-modify, or imagine conversations with women in their mind.
Jagadananda Pandit writes in Prema Vivarta,
svapane o nā kara bhāi strī-sambhāṣaṇa
gṛhe strī chāḍiyā bhāi āsiyācha vana
yadi cāha praṇaya rākhite gaurāṅgera sane
choṭa haridāsera kathā thāke yena mane
“Do not think of women even in dreams. You have accepted the renounced order of life with a vow that forbids you to associate with women. If you wish to associate with Caitanya Mahāprabhu, you must always remember the incident of Choṭa Haridāsa and how he was rejected by the Lord.”
Not only conversing and talking, but even thinking has to be given up. When one is wanting to think about women and the conversations that one has had with them, or one desires to have with them in future, that is a clear indication that the mind is infected by thoughts of sense gratification.
When we stop this kind of thinking, then only we can think of doing some positive service to Krishna.
As Srila Rupa Goswami says in Bhakti Rasamrita Sindhu,
bhukti-mukti-spṛhā yāvat
piśācī hṛdi vartate
tāvad bhakti-sukhasyātra
katham abhyudayo bhavet
The material desire to enjoy the material world and the desire to become liberated from material bondage are considered to be two witches, and they haunt one like ghosts. As long as these witches remain within the heart, how can one feel transcendental bliss? As long as these two witches remain in the heart, there is no possibility of enjoying the transcendental bliss of devotional service.