I think I've boiled down a few themes behind the thought processes people that hate crossdressers.
1. Pity. You are a failure as a man - the dominant gender - and that is why you chose to retreat into the passive gender.
2. Disgust. For example, a male might be offended that he is in some way attracted to you. Or a female may think, what a shame, he was such a handsome man and now he's such an ugly woman.
3. Insult. A male might see a crossdresser as making a statement that being female is better than being male, thus categorically insinuating that the male sex in general is inferior. A female might see a crossdresser as making a statement that femininity originates in the clothing and not in the heart.
Any others?