SB 9.20.16
om ity ukte yatha-dharmam
upayeme sakuntalam
gandharva-vidhina raja
desa-kala-vidhanavit
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When Sakuntala responded to Maharaja Dusmanta’s proposal with silence, the agreement was complete. Then the King, who knew the laws of marriage, immediately married her by chanting the Vedic pranava [omkara], in accordance with the marriage ceremony as performed among the Gandharvas.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The omkara, pranava, is the Supreme Personality of Godhead represented by letters. Bhagavad-gita says that the letters a-u-m, combined together as om, represent the Supreme Lord. Religious principles are meant to invoke the blessings and mercy of the Supreme Personality of Godhead, Krsna, who says in Bhagavad-gita that He is personally present in sexual desires that are not contrary to religious principles. The word vidhina means, “according to religious principles.” The association of men and women according to religious principles is allowed in the Vedic culture. In our Krsna consciousness movement we allow marriage on the basis of religious principles, but the sexual combination of men and women as friends is irreligious and is not allowed.