The word phobia, meaning “irrational fear, horror, aversion,” entered into English in 1786.
The use of phobia to describe an anxiety disorder in the psychological sense began about 1895.
About 1800, -phobia, a suffix or word-forming element meaning “excessive or irrational fear of” began to be used to form words describing specific fears.
So the original meaning included aversion. And even if it didn't language changes. Awful use to mean worthy of awe, naughty used to mean someone who has naught. Language is agreed on meanings. If you know what people are trying to say and choose to use some perceived linguistic faux pas to gatekeep, you are obviously just trying to short circuit the conversation. Or , put another way, making a big deal about how someone is saying something instead of looking at what they are saying is elitist.
Why would I need to change words? We have an awesome language with so many existing words. Intolerant. Discrimination. Heteronormative. Bullying. How about heterosupremacist - I just made that one up, so it can't possibly be misused.
So if I thought everyone who said anything about trans people or in disagreement with me was an asshole or -phobic, you'd be right. But I don't. I can understand concerns over transgender youth in sports. I can even understand concerns about transgender people in restrooms, though I think the risk is blown up, and based on the almost total lack of occurrence, not a concern I have. You can respectfully disagree and that is cool.
But mocking people with gender dysphoria which is either a mental condition or mental illness (depending on who you ask) is an asshole thing to do. Transgender people have suicide rates far above average, and that rate seems to be present whether they get confirmation surgery/hormones, live as their experienced gender without any sort of gender confirmation, or if they try to live as their assigned gender. Is it the bullying and mockery or something else? Its hard to say. So yes, mock somebody for being transgender and you are 100% an asshole - full stop.
Now if you want to discuss whether it is appropriate for children to be able to get hormone blockers or the like even with their parents permission, or transgender kids in sports, or any of a number of interesting questions that have moral implications without being all about mocking someone, then that's all good.