It is intended as an insult. However, I always thought narrow minded meant that you only looked at something from a narrow view as in you only saw one side of an argument and either did not or would not look at it from another point of view. The problem with this insult though is that someone can analyze a situation from multiple points of view and still feel their original perspective/solution was correct. Unfortunately, if they are not good at articulating why they arrived at the conclusion, they are easily labeled as narrow minded.