If you spend your book using your character to preach your ideal of utopia, most of your readers will feel ill to the stomach. Why? Because most so-called utopias in fact could never work, and many readers will be educated enough to know that.
Stick to reality. If you make a future where even the dirt is clean because people gave up on violence, or the whole book is geared toward bashing gun ownership because Bambi died, or pushing your own personal political agenda, your readers (both for and against your stance) will get sick of it real fast. Your job is to just tell a story, not revolutionize the world. Head for nonfiction if you want to do that.
Preaching is dangerous because it tends to lead into very long, dry, boring expository lumps. (Read this Expository Lump article by Aneurin for a detailed and very amusing explanation of what an Expository Lump is!) That is, a long boring text-book like section of writing that has nothing to do with the story, but has everything to do with the author’s own research, knowledge and opinion.
Either that, or if the whole story is set up around your pet peeve, you will annoy the ones who agree with you and disagree both.
Be realistic, do research: if it hasn’t happened that way in history, don’t try to make it happen in your story. If you preach, you are centering yourself on a single issue; you are either preaching to the choir and they will skip it, or you are preaching at the opposition and they will get annoyed. The point of Fiction is that it is NOT here-and-now, and you need to take the here-and-now out of the equation.
An example of ruining a story by preaching is Frank Herbert’s “Dune.” The first part of the first book is obviously pure inspiration, and is the best part of his entire series. It is, in fact, the only real bit of story he wrote. However; very soon he left his inspiration entirely behind, and began to preach. Impressed by Muslim culture and theology, he took his little Messiah character into the Arabian Desert (called Dune) and played out his utopia of a Muslim redeemer, based on religious ideals... NOT on his original inspiration. He betrayed his muse.
On the other hand, George Lucas was also inspired by Muslim theology when he created his Jedi. (The Arabic word for divinely-inspired fighter with supernatural powers is ‘Jeddah.’) But he allowed the story to run on its own, grow and change beyond recognition, until it became cool. You can be inspired by something, but if you hold it in a stranglehold and tether the story to an idea, you will smother the story.