In polite company, it is appropriate to talke about descriptive or factual topics, like the weather, your job, or your kids/family. Those are not very controversial.
Sex and religion and politics have judgements interwoven with them. They define us at a much deeper level than what we do for a living or what the weather is like outside. Religion determines what is right and wrong. Sex is taboo in all but a monogomous married environment and indirect judgement is passed on those who violate that taboo. Politics is the effort to impose the will of the majority on the minority.
Revealing how you feel about any of these can wipe out previous opinions about you. You may be held in high standing in society, until it is found out that you are sleeping around, or you take an unpopulare political stand. You not only place your beliefs out in public for comment and ridicule, but you also indirectly strike at those who believe something different.
For some reason, humans take different opinions about these subjects as a personal attack on themselves at first. Unless a requirement is established at the begining of the conversation to remain calm and uninsulted during the discussion, the discussion usually degrades into hurt feelings or worse. Even then, constant reminders are needed so that statements of opinion will not be taken as personal insults.
Inteligent and rational people can discuse these topics calmly, and agree to disagree, but that is a rare combination in the world today. Most people are missing one if not both of these traits. Online, there is very little encouragement to act in a civil matter toward one another, so behavior on Askville should not be the yardstick for these discussions.
I can discuss these topics calmly without being insulted, but I avoid them with all but my closest acquaintances since I do not know if others can do the same.
I hope this helps.