When the Left say gender identity is different from biological sex, what they mean is that it is perfectly legitimate for your mind to exist in a state of delusion. (In other words, it is not important that your thinking is aligned with reality.) This means that transgenderism, which promotes the idea that gender identity is different from biological sex, promulgates severe mental illness and its acceptance, through social contagion and other means. Ergo, given that the purpose of transgenderism is to bring about mass psychological harm to society, you begin to see that transgenderism is actually a weapon of mass harm, rather than a movement aiming to achieve good, as the Left claim that it is.