The answer to this depends to a large degree on the cultural norms of the society in which you live.  Even within a country like the United States, there can be different cultural norms among different groups.  This makes it difficult to say for certain that a given topic would be “unsafe” or “inappropriate” without knowing about the specific situation.  However, there are some topics that are more likely to be considered “unsafe” by many people in the United States.

First, there is the subject of sex.  Among many people, this is considered to be a private issue that should not be discussed in public.  This becomes particularly true when the social situation includes people of both sexes.  It is even more true if there are children around.  Second, there is the subject of politics.  Finally, there is the subject of religion.  These last two subjects are not necessarily seen as “inappropriate” in the way that sex is.  However, they would be seen as “unsafe” because of the great possibility of causing conflict.  People have such divergent views on politics and religion that any discussion that centers on those issues can easily end up with people getting angry at one another.  This is not what is wanted in a social situation.