I know why it's so difficult to remain civil. To take the emotion out of a controversial subject and instead look at it objectively requires a great deal of mental effort. It's not easy for any of us, and it's even harder if you personally are directly affected by the subject being discussed (example: The topic is welfare, and you grew up on welfare...or the topic is abortion, and you had an abortion). I know when I sometimes hear a mean spirited comment about the people who voted for George W. Bush twice, I flinch. My parents are two of those people, and they're smart and decent individuals. I have to control my response.
Once emotion kicks in though, you (meaning: any one of us) feel justified in your anger, and the sharps words you use seem justified as well. It's NOT right, and it's never helpful, but it feels right.
I personally have gone out of my way to practice objectivity. It has taken years, but I feel I'm getting pretty good at it now, and I can take nasty hits with a decent degree of grace (got a LOT of practice while participating on an abortion/religion forum). The up side is that I get to enjoy the more cerebral aspects of these discussions, and my feelings are not easily hurt, which gives me a bit more freedom in the conversation (as opposed to my husband, who instantly gets angry at the first offensive tone, so the rest of the discussion goes downhill). I also learn a great deal.
The downside is that I sometimes push others to approach the conversation the way I do, encouraging manners and respect, which has unintentionally earned me more than one enemy. *shrug*