In this material world, sexual engagement is considered to be the highest engagement, the greatest pleasure, although it exists only in a perverted form. No one, however, has conceived that there can be sexual engagement in the spiritual world. There is not a single instance of such theology anywhere in the entire world. Caitanya Mahaprabhu gave the most perfect information of our engagement with the Supreme Lord. In other religious conceptions of life, God is at the utmost taken as the father or mother. However, Caitanya Mahaprabhu informs us that one can even have sexual engagement with the Lord. This information is Caitanya Mahaprabhu’s unique contribution. This information is given for the first time by Caitanya Mahaprabhu: one can have the Supreme Personality of Godhead as one’s husband, as one’s lover. This is possible in the worship of Radha and Kṛṣṇa, but no one, especially the impersonalists, can understand Radha-Kṛṣṇa. The impersonalists have no idea; they cannot even conceive that God has form. But Caitanya Mahaprabhu says that not only does God have form, but He has sex life also. This is the highest contribution of Caitanya Mahaprabhu. According to Srila Viswanath Chakravarty Thakur the entire material world turns due to the basic principle of sex life. In the modern human civilization, sex is the central point of all activities; indeed, wherever we turn our face we see sex life prominent. Consequently, sex life is not unreal; its true reality is experienced in the spiritual world. Material sex is but a perverted reflection of the original; the original is found in the Absolute Truth. This validates the fact that the Absolute Truth is personal, for the Absolute Truth cannot be impersonal and have a sense of pure sex life. The impersonal monist philosophy gives an indirect impetus to abominable mundane sex because it overly stresses the impersonality of the ultimate truth. The result is that men who lack knowledge have accepted the perverted material sex life as all in all because they have no information of the actual spiritual form of sex. There is a distinction between sex in the diseased condition of material life and sex in the spiritual existence.
Spiritual sex is of two kinds: one in accordance with the constitutional position of the self and the other in accordance with the object. When one understands the truth about this life but is not completely cleansed of material contamination, he is not factually situated in the transcendental abode, Vrndavana, although he may understand spiritual life. When, however, one becomes free from the sex urges of the material body, he can actually attain the supreme abode of Vrndavana. When one is so situated, he can utter the kama-gayatri and kama-bija mantra.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “The Science of Self-Realization”, Page 333
A.C. Bhaktivedanta Swami Prabhupada (2012 edition), “Teachings of Lord Caitanya, The Golden Avatara”, Page 288 & 362