It could happen.
People have died in defense of marriage. Could it happen today? I thought about this after reading today's Gospel. It is the story of Salome, the seductive daughter of Herodias, whose dance aroused Herod's passion, moving him to offer her half of his kingdom. At her mother's prompting, the girl asked for the head of John the Baptist instead. Herod's illegitimate wife hated John since he had condemned their marriage pointing out, "It is not lawful for you to have your brother's wife." [Mk. 6:14-20] Herodias looked for a way to have him killed after that.
A more modern example of the faithful dying in defense of marriage - amongst other truths of the faith - would be the English martyrs, Thomas More and John Fisher, who died under Henry VIII. Kind of a stretch? Not really, they died because they would not take the Oath of Succession. They refused to take the oath because it denied the authority of the Pope, while supporting the illegal annulment of the marriage of the king to Catherine of Aragon.
Today's first reading from the Letter to the Hebrews instructs us, "Let marriage be honored among all and the marriage bed kept undefiled, for God will judge the immoral and adulterers." [Heb. 13:1-8]
Art Upper: The Beheading of St John the Baptist c. 1869 - Pierre-Cecile Puvis de Chavannes.
Art Lower: Feast of Herod, 14th C. - Master of the Life of St. John Baptist