What I suspect they are offended by is the idea that all the world's faith's are equally valid and like spokes of a wheel all lead to the Creator of all things but merely represented differently in various cultures. Certainly, that is the underlying message by those who sport these bumper-stickers on their automobiles. I agree that there is only one way leading to God because God Himself has made that abundantly clear. However, the concept of "coexist" is fundamentally Christian and also a direct command from the throne room of heaven to God's people living on this Earth. Below is the best characterization of this command:
"To the degree possible and whenever you can, live at peace with all people."
~ Romans 12:18 (Kimicus ad Aburdum translation)