Use Common Sense.

Beyond politics and religion, some things just aren't polite for conversation. Use common sense: if someone says something, and it strikes you as off-limits, tell them "Hey, common sense suggests that's just a rude topic of discussion." You can use common sense in a lot of other ways, too, to imply that your opinion is either valid generally, or derived from some upwelling of Jungian shared psyche. Common sense should always be used.