So was Jesus being rigid when he called out the Pharisees for holding to Moses' policy of granting a bill of divorce?  Was St. John the Baptist rigid when he told Herod that it was not lawful to take his brother's wife?  Were St. Thomas More and St. John Fisher rigid when they didn't go along with Henry VIII?

All Cardinal Burke is asking is if there are still objective moral norms, and it's still a sin to have a sexual union with one person while married to another, something Our Lord Himself labeled adultery.