Let's say I think ethnocentrism is morally correct. Why is your opinion that it's morally incorrect true while my opinion is incorrect?
Morals are just opinions yo
Let's say I think moral relativism is morally correct. Oh wait, I can't. Because moral relativism says that morals are opinions, so technically my own moral stance that morals are relevant to the individual is also an opinion. I'm neither correct nor incorrect, but I'll still teach this standpoint in my philosophy class because ethnocentrism isn't pollitically correct, and just saying women shouldn't be able to kill their young or that african tribes shouldn't practice female circumcision is clearly the wrong thing here.
People are perfect, clearly, so why would we need to impose our morality on them? Why are we so intolerant of other people's opnions or ways of life? We're too righteous and judgemental. Morality is a private affair. We should keep our nose out of other people's business.