I'm agnostic, so I debate with everybody. The reason it seems atheists and such are argumentive is because those that follow a religion assume that they are correct. Most atheists don't say anything about small items. But when they are consistantly having these beliefs thrown in their faces, they eventually feel a need to say something. I'm usually quiet about such topics until I have been bashed over the head with religion enough times. The only reason you seem them as argumentive is they don't believe what you want them to. To me, Christians are argumentive.