Just before my friend told her boss that she was a transwoman, she had the experience of being in the cafeteria with the TV on when a particularly masculine looking woman came on a talk show. Everyone around her started giggling and cracking jokes, and generally insulting trans* people - never realizing that about a month later their co-worker (and in some cases supervisor) was a transwoman. That is what trans* people experience every day - even after transition.
So when you talk about Chelsea in a way that is not respectful of her expressed gender preference, you are sending the message to every trans* person who reads it that they don't deserve your respect. In the context of someone who has expressed a preferred gender pronoun, s/he is extraordinarily offensive because it is too close to one of the main words people use to dismiss trans* individuals - "he-she," a word of ridicule. The reaction was probably stronger to that particular word choice than had you merely used masculine pronouns.
You must have had at least one experience of sitting in the room hearing yourself discussed in the form of someone else with whom shared a characteristic that was distasteful to the speaker. That is what the reaction was about. Even though you may have thought you were talking only about Chelsea, the way you talked about Chelsea really sends a message to all trans* individuals who read it.
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