What does it mean to say that something is a matter of opinion? You could mean
1) Claims about subject matter X can neither be true nor false absolutely. Thus, a typical matter of opinion in this sense would be the question of whether McDonald's burgers are better tasting than Burger King's. There can't be any sense of saying that someone erroneously believes that McDonald's burgers are better when they are not, since each person's personal taste decides it for them. This would be a personal, as opposed to a cultural relativism.
Or, you could mean
2) Rational persons can disagree about subject matter X. An reasonable person, examining all of the relevant evidence, could decide the question either way. So, for example, with the question of whether there is life on other planets equivalent in intelligence to our own, many people think that we are not in a position to settle the question for sure one way or the other. However, the question has a true-or-false answer, but we are not in a position to determine which, because we don't have enough information. The question of whether or not there is a God is sometimes thought of being a matter of opinion in sense 2, but most would agree that either there is a God or there isn't.