A sannyasi should not take part in a marriage ceremony between a young boy and a young girl. However, this is not a karma-kanda activity, because our purpose is to spread the Krsna consciousness movement
“Lord Kṛṣṇa is very much pleased by service rendered to brāhmaṇas, and when the Lord is pleased, the opulence of one's devotional service increases.”
In this regard, Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura comments that the younger brāhmaṇa rendered service to the older one with the purpose of pleasing Kṛṣṇa. It was not a matter of ordinary worldly dealings. Kṛṣṇa is pleased when a Vaiṣṇava is rendered service. Because the younger brāhmaṇa served the older one, Lord Gopāla agreed to become a witness of the marriage negotiation in order to maintain the prestige of both devotees. Śrī Caitanya Mahāprabhu would certainly not have liked to hear about marital dealings unless such dealings were exchanged between two Vaiṣṇavas. Marriage arrangements and ceremonies belong to ordinary material karma-kāṇḍa sections of the scriptures. The Vaiṣṇavas, however, are not interested in any kind of karma-kāṇḍa dealings. Śrīla Narottama dāsa Ṭhākura says: karma-kāṇḍa jñāna-kāṇḍa kevala viṣera bhāṇḍa. For a Vaiṣṇava, the karma-kāṇḍa and jñāna-kāṇḍa sections of the Vedas are unnecessary. Indeed, a real Vaiṣṇava takes these sections as a poison pot (viṣera bhāṇḍa). Sometimes we take part in a marriage ceremony for our disciples, but this does not mean that we are interested in karma-kāṇḍa activities. Sometimes, not knowing the Vaiṣṇava philosophy, an outsider criticizes such activity, maintaining that a sannyāsī should not take part in a marriage ceremony between a young boy and a young girl. However, this is not a karma-kāṇḍa activity, because our purpose is to spread the Kṛṣṇa consciousness movement. We are giving all facility to the general populace to take to Kṛṣṇa consciousness, and in order to fix the devotees in concentration on the service of the Lord, marriage is sometimes allowed. We have experienced that such married couples actually render very important service to the mission. Therefore, one should not misunderstand when a sannyāsī takes part in a marriage ceremony. Śrī Caitanya Mahāprabhu and Nityānanda Prabhu took great pleasure in hearing about the marriage ceremony between the young brāhmaṇa and the daughter of the elderly brāhmaṇa.