Well, because the pastor may have his own views on marriage and nullity, same with the views of the first spouse (with or without child(ren)), the DRE who may catechize the kids. None of them may have the authority of he magisterium to buttress the stance.

All the talk about changing norms in society calls into question the Divinity of Christ. He knew some of His teachings would be hard to take, let alone understand. He also knew, because He is omniscient, that divisions would appear, and yet He did not offer ways around these hard teachings.

We believe that He gave us the Holy Spirit to guide the unerring Church, the Divine Church, not its flawed human components. Not everything is doctrinal, but rather prudential, with leeway on some issues.