“Homosexuality is unnatural. This is scientific fact—not an opinion.”
Here is a simplistic illustration of the problem with the teaching method: “Homosexual acts are wrong” - fact or opinion? To call it opinion (as they would no doubt call it, and a “wrong opinion” at that) is to assume that there is no one with the authority to declare it so, to elevate it to the level of fact.
Oh, I agree the teaching method has a serious flaw. I’m sure some would say that “homosexual acts are unnatural” is not only an opinion, but a wrong one. To that, I’d simply reply that they are wrong. If everything is opinion, who are they to say I’m wrong? Their own methodology calls into question everything they believe.
Of course, saying everything is opinion ignores the fact that some opinions are demonstrably true. If I say ingesting 10 grams of polonium will kill you, that’s just an opinion, but it’s also true. From Wikipedia: “One gram of 210Po could thus in theory poison 20 million people of whom 10 million would die.” We could argue over probabilities, but the odds of survival are so small as to be irrelevant for all practical purposes.
It goes beyond even this. Opinions can be true without being demonstrably so. Love exists, but try proving it.