Many of the things I thought of when I first heard this thread name have already been covered. I still have some other reactions to the topic, though.
First of all, I think the question Alan really wanted an answer to is "What can you not perform songs about?". Most of us who are songwriters and/or poets have written a lot of things we would not consider performing. Some because they are just not really good songcraft.
One of the ways you get the good ones is by going ahead and writing whatever comes through, if only for the practice.
Also, some songs won't let you NOT write them.
I wrote one a few years ago about a friend's husband, who went down in the basement one day and blew his brains out. I have performed it exactly once, and will likely never do so again. But it needed to be written. I needed to write it, because putting it into the framework of a song/poem was the only way I could deal with the issue.
I have, over the years, written many poems and songs about issues that make people uncomfortable in themselves. In part, the writing is a personal catharsis. But almost every time I perform one of them, someone comes up to me later and thanks me for reminding them they are not the only one to feel that way. Often I've been told that I put words to something thay had been unable to say for themselves. Or that they now feel differently about an issue for having heard another side of it.
Whether or not to perform a given song is a personal decision, and should take both audience and message into account. Whether or not to write it? Doesn't your creative self have enough blocks without adding more? Besides, that very creativity is what can keep you alive in extreme situations. I know several songwriters who have written very painful songs about pain, seperation, loss, death, or illness in their lives. Often the very act of writing the song is what helps get you through that stuff.
I would say, go ahead and write! Edit later.