All teaching is based on personal opinion, though. How else can it be done? What led to the first world war? Was X Y or Z a good King/PM/War leader? Is a sonnet a pure form of poetry or a restrictive one? The answers to these questions are all personal opinions, but worthy of consideration, surely?
In the end, the point of education is (or should be) that children learn how to work out the answers to these things on their own, but they need to be aware that there are different points of view, and crucially what is the difference between a prejudice and an educated opinion.
As always, it is dangerous to have a knee-jerk reaction without knowing the detail. Was there a discussion about the comments? Were they presented as facts or as the basis for debate? Was another point of view given?