First I would like to say that I have been against the inclusion of some controversial elements in the past, mainly questioning what they would add of substance to the game.
My argument being: in what way do the benefits outweigh the problems these topics present, is there any way the DF community can handle them, and in many cases "why can't we just pretend?"
I believe that I now understand the purpose of DF as a simulator for a "real" fantasy world, and that while it is possible for a brilliant individual to create such a world, dictate its rules, tell a story therein and even share it, DF is meant to allow anyone to actually participate in such a world free of the encumbrance of any creator or GM! This, I believe, is the true brilliance of the scope of the idea of Dwarf Fortress, and is what leads to all of the wonderful side effects like unpredictable events that we often mistake for the genius itself.
Any such project, or "fantasy-reality simulator" as it might be called, must necessarily include all elements where possible, good or evil, disgusting or mundane. Incest, slavery, violence, petty individualism, patriotism, nationalism, and pagan religion included. While we might pretend any part or even the whole of Dwarf Fortress, its purpose and the inclusion of these elements cannot be supplanted by mere pretense and imagination. DF's job is to bring them to life, to make them real for the player. They are indispensible.
I do, however, believe that many things should be left to be "manually enabled" by the player, and should not be default settings.