I'd say it's a valid opinion, though based on ignorance. When you grow up indoctrinated by your environment you'd have a hard time losing your bias on your own.
If you'd only have met mean people of a certain 'group' in your life it's understandable to form a negative opinion based on that. Even if you know that this might just be bad luck on your part it's still your experience.

It comes down to one's willingness to remain open minded and actually learn differently. One shouldn't condemn others based on that but it's understandable to be biased and 'careful'.
I have a bias regarding many classic right wing opinions and a hard time to not assume that a person with a strong opinion on a certain topic also ticks off all other related opinions. While I think my bias to think of any Republican or Libertarian being right-wing is valid, based on my experience, it's still a bias and won't hold true to every individual I 'classify' this way.