I would argue that an opinion:

1. Which is poorly considered and researched AND
2. Which is likely to do harm through your holding it AND
3. Which you should know will have important consequences, dependent on whether it is true or false.

Is immoral to hold. In other words, it is immoral to hold beliefs with important consequences without considering such beliefs.

More exactly, it is the fact of the opinion being poorly considered and researched which is immoral- so strictly speaking the opinion isn't immoral, but the process that leads to it is. It is commonly thought that doxastic states (states of belief) are involuntary. This may be true (I'm honestly not sure), but one can control one's choices to research a topic, spend time considering it etc.

For example, if you believe that Italian people are dreadful, and as a result refuse to vote for any Italian candidates, give your vote to anyone who promises to stop Italian immigration etc. you have an opinion which is doing harm through its effect on your behaviour. If (as seems likely) you haven't bothered to research it or consider it, despite knowing that such a belief will have important effects, it seems to me that you are behaving unethically.