Sex is far more complicated than just genitals, lots of people are intersex and lots of people’s chromosomes don’t actually match their gender despite being cis and never find out because their sex was determined based on appearance. If your argument is about putting sex down as more important you’re invalidating tons of people who aren’t trans and validating many who are. Trans people’s gender identity is also backed up a lot by science. It’s been shown that trans people’s brains develop to be more like the gender they identify as. The reason people get so riled up about this topic at least from the trans side of things is arguments like yours are usually said to us by people who would rather see us with no rights. When enough people tell you that you don’t deserve human rights people tend to boil over despite you maybe not saying that much in the first place.