Exposure.
I'm no shrink, nor do I play one on TV ... but all that Victorian, Freudian stuff scares the Crap out of most people.
The biggest lies America ever bought into was .. "Never discuss religion, politics nor sex" .. and yet ... those seem to be the very areas a lot of people are freaked out about.
I think those conversations that are difficult are the ones that expose us to admitting something .. not necesarrily saying a particular thing, but the flavor of our conversation may reveal, "Ohhhh ... you really like tiny boobs" .. or .. "I didn't know you had those socialist thoughts" .. or .. "Wait a minute .. are you implying baptism is not necessary for salvation?"
All this exposes us to have to dig deep into areas that, perhaps, we haven't completely digested yet .. and .. rather than risk being questioned, we refrain from the topic altogether.
The big three.
When I started this thread, I didn't consider the attitude that religion, politics, and sex should not be discussed in polite conversation. (Some versions include money in that list.) They provide an interesting place to start, but I had always seen different reasons to avoid those subjects.
I had always seen religion as taboo because most belief systems define themselves as the only way to reach God. If one truly believes one of those systems, then there is a kind of condemnation of anyone outside that system. I think mature people can learn to be friends in spite of that difference, but most of us don't like having to state that a friend is condemned under our belief system and don't like being seen that way by a friend.
Politics is similar except that the condemnation is in this world instead of in the next world. Someone who wants to take my money (or allow others to take my money) for wrongful purposes is my enemy. Someone who wants to take my freedom without just cause (as I see just cause) is my enemy. I'm less worried about exposing my belief in the sense of letting someone know what I believe than I am about having to define someone as my enemy.
I had always seen sex as a forbidden subject because our society has traditionally been much more modest. The sexual relationship between a man and his wife was only for the two of them to understand. Discussing the topic was considered immodest and an invasion.
I'd like to hear others' opinions of what makes a forbidden topic into a forbidden topic.