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Imagine you are a closeted transperson, and you go online to talk with people about your issues. Then one day, some major web corporation decides that real names are the way to eliminate online trolling, and retroactively attaches your real name to one of the accounts you were using to maintain your presence. Now all of a sudden, your entire family learns of your condition, when you had no intention of telling them at that time. Worst case scenario, you're a teenager or other dependent that wasn't planning to tell your family until you could support yourself, and your parents decide to kick you out of the house, making you homeless, or force you into therapy and onto antidepressant medication that leads to your suicide.
Because of these very justifiable fears, transpeople can be among the strongest proponents of privacy and online anonymity.