Subjectivity: Only I can know if I am being as good as I can be?The second argument is just stupid and while we all make it absent-mindedly at some point in our lives we only need to be reminded that not only we actually do make moral judgments all the time, we make moral judgments that we all agree about all the time. Yes, it is true that there are grey areas where we might have a hard time agreeing but there are so many more areas where we do agree.
Blunt relativism: No one can know, so we never can judge.
But what about subjectivity? Subjectivity is a big thing in a lot of current Catholic moral theology debate. The argument might be put like this: Only the person living their life can really know what it is like to struggle against what they are struggling so no one else can judge for them.
Is that true? I don't think so.
It is true that we often fail to understand others but we also often succeed. Men can understand how it feels to be a woman and women can imagine what it feels like to be a man. We may not get it exactly right but we can develop our empathy such that we can grasp what it is like. And yes, we can grasp how people of other races and cultures feel and why they feel that way.
It's not scientific knowledge, it's not like knowing how to navigate from Ithaca to Bethesda, it is not like knowing how to generalize the Pythagorean theorem for four dimensions and it's not like knowing how to make a Sidecar. But it is something human beings can and do do.