τοὺς υἱοὺς τοῦ νυμφῶνος. This is a Hebraism for the friends of the bridegroom—the paranymphs—who accompanied him to meet the bride and her maidens; Jdg 14:11. The question would be specially forcible to John’s disciples who had heard him speak of “the joy of the friend of the bridegroom” (Joh 3:29).
νυμφῶνος. Compare the words παρθενών, γυναικών.
ὁ νυμφίος. The term implies a fully-developed Messianic consciousness in the speaker (Hos 2:19).
νηστεῦσαι. St Matthew (Mat 9:15) uses the word ‘mourn’ which makes the antithesis more striking (Joh 16:20).