Consensus obviously cant mean every single person agreeing, its about what the widespread view in the culture is.
Either way its a hypothetical, doesnt matter if such a culture never existed in reality: suppose slavery was condone by some culture. Wouldnt that have made it moral?
Going by the meme: if a society is mysognist you would be wiling to agree its correct for them and womens rights activist in that society should stop (theyre going against what the culture has decided is moral, making the activist immoral)?