I Think being offended by something and understanding why something is offensive should align; I feel like once you understand why something is offensive then you have an ethical imperative to find that thing offensive. The further apart those two things are, I feel, correlates to an increasing lack of empathy.
I think I now understand your point, and how you could arrive at it, but I don't agree with it.
First of all, where does this "ethical imperative" come from? Does my act of getting offended about something that happened to somebody else make a positive difference in the world, or does it just make it so that one more person is less happy than they were earlier? I can't see how getting offended over something is in any way ethically right. By definition: offended-resentful or annoyed, typically as a result of a perceived insult. How could me getting resentful or annoyed in any way be a good thing? I think what would be a lot better for the world is if, instead of wasting time and energy on being offended, I instead stayed calm and simply took some sort of action to help the repressed group. Perhaps that could be donating, perhaps that could be writing a supportive comment, perhaps I take part in a protest, etc.
But being resentful or annoyed? Isn't that more likely to just lead to me saying something offensive back?
So, if we take this idea of an "ethical imperative" out, why should I find something offensive just because I understand why someone else would? Some people are uneducated, some people are ignorant, some people are crazy, some people are misguided, some people are easily offended, some people are faking offense for attention.
I understand why extremely religious parents took offense to the Harry Potter books. They are extremely devout in their beliefs, and take offense to anything that in any way contradicts or contrasts them.
But should I get offended at the Harry Potter books? No. In fact, I enjoyed reading them.
So you can see now that there has to be a subjective line on what we should get offended at, doesn't there?
Otherwise I would both simultaneously have to be offended at the Harry Potter books, and the people who were offended by the books, and the people who were offended that people were offended by those books, which now includes me, which means I'm offending myself.
So in the end it seems like it just comes back to what you personally find offensive. Perhaps you still disagree, in which case I guess I'll have to agree to disagree in advance.