As I've said a thousand times, we Protestants had people like Fred Phelps. Sad but true. But even the most hardcore fundamentalists I knew wouldn't get near Phelps with a 10' pole.
Yet Mark is frequently picked as a contributor to, or speaker at, Catholic publications, Catholic podcasts, Catholic seminars, Catholic parishes and Catholic colleges and schools. See the difference? That's one point for Protestant discernment.
UPDATE: As if to demonstrate, I saw this link where Mark goes after his favorite undesirables, the white conservative Christian with white skin, preferably of the male variety. Nothing really to bother with, any more than I would go to Westboro Baptist and try to reason with them. It's enough to make people aware and then move on. There are some people who are beyond reach on this side of heaven. All that's left to do is prayer and fasting.