Given the dictionary definition of opinion you posted of an opinion being "a view or judgment formed about something, not necessarily based on fact or knowledge", how so? Your definition of opinion perfectly describes this situation yet you say it isn't an opinion. Do you have a different definition of opinion that would preclude a view or judgement which this clearly is?
I'm also curious as to how you think it could be correct or incorrect. By what metric would you judge correctness or incorrectness?
Most adults who saying eating meat is moral, and most who say it is immoral, are basing their decisions on facts and knowledge.
You would judge by the metric of whether it is a conclusion to the correct set of of initial assumptions about the basic moral principles, contrary to them, or undetermined based on them but still with a moral value of it's own.
Let's go with something a little simpler and less likely to be controversial. Our commonly used mathematical system is based on a logical framework where every well-formed statement is either true or false. All of our commonly used are based upon adopting a set of 8 or so (depending on the construction) axioms/axiom schemas, which are chosen for seeming obviously true to us.
However, there will always mathematical statements that can't be proven true or false based on any non-contradictory axiom schema, and sometimes there will be statements that seem like they have to be true, and yet have consequences that are so absurd they seem obviously false. By the system we have set up, these statement have a real value of true or false, but we have no way of determining what that value is.
Given that this is true in something as simple as mathematics, I'm quite comfortable with the notion that in a more complex field like ethics, while every action that has a impact on others that they notice has a moral value, we may not be able to determine what they are. Our inability to authoritatively assign a value does not make every possible assignment equally valid.