Heathen's question

Both parties' candidates have been attacked for being present in church while particularly offensive or controversial sermons were being given. As a non-churchgoing person, I have been puzzling over exactly what this is supposed to mean. Do you know the content of a sermon in advance (beyond a title, perhaps)? If you find it deeply objectionable, are you supposed to walk out? Does anything like that ever actually happen? Or do you just sort of fidget uncomfortably and then talk trash about the minister at brunch?