For me, it's three fold.
First, there is the desire by the creationist side to get their religious beliefs taught in public schools. Evolution is science, and deserves to be taught in science class; creationism (of all types) is religion, and should be taught in Sunday School.
Second, I am sick to death of being call evil, immoral, a Satanist, stupid, a Communist, a Nazi, a Leftist, a Democrat, a sex pervert, a papist, or having something against Christianity because I understand that Evolution is a fact.
Finally, I find willful ignorance offensive. I am not saying that people who believe in creationism are all willfully ignorant, I am talking about the subset who, no matter how many times you point out the evidence, insist that there is no evidence. It's friggin' Orwellian. If you choose not to believe in evolution, that's fine. I can respect someone saying, "I value my faith over science" or "I choose to disbelieve the science on religious grounds" or "I am rejecting the evidence because I believe I have a command by God to do so." But don't sit here and blow smoke up everyone's asses by saying that there is no evidence.
Thank you for your reply. It was well said and I appreciate you taking the time to explain it to me. I just find that both sides of the argument have supporters that can be a bit overzealous in their defense of their positions. Whenever I find someone who gets so emotional about an issue, I wonder what exactly their motivation is.