I don't mind being referred to as a fundamentalist when it is just meaning this:
From the Christian perspective, fundamentalist has traditionally referred to any follower of Christ who believes that the Bible is the inspired Word of God and who believes in its literal interpretation and fundamental teachings. The fundamental Christian believes in the experience of the "new birth" which occurs when faith is placed in Christ as Savior and Lord. To the world this may be viewed as radical, but is very basic to the Christian faith
After all that is how I live my life, and how my faith works. But when "fundamentalist" is purposely used in a derogatory form of attack, then it becomes offensive.