This is a pretty classic mistake. You see, words and their definitions evolve. In this instance, gender and sex are actually two different things. Sex, here, is the biology--the chromosomes, as well as primary and secondary sexual characteristics. This is what you're referring to.

Meanwhile gender is a complicated cultural construct--made up of everything from clothing to speech patterns to pronouns. While they can be, and often are, related they are not the same thing.

Be kind to trans people. They get a lot of shit and kindness costs you nothing. In fact, I'd wager that your anger is making you an unhappier person, whether or not you realize it.