If you can tell me you're offended, you can tell me why. I don't think that's too much to ask or even slightly unreasonable.
If I tell you I'm offended, best believe I have a reason at the ready. I had a discussion about the word "midget" with a woman who, although she didn't personally take offense to it, explained that others with dwarfism could and why. Good discussion, and although it slips out from time to time (I'm damned near 50, cut me some slack), I make a conscious effort not to use it.
That's what I expect. Saying that someone may not have the language to describe something is a cop out. It can be explained in simple terms and in many cases it's probably best to explain it in simple terms. I once witnessed a kid with Down's Syndrome explain to an old cat why calling his group "retarded kids" when asking their teacher about them was offensive. If he could do it with his own limited vocab, I can't really excuse anyone else's inability to tell me why they're offended.