The concern seems to be about defaults. I.e. what options are we offering to children and young people who are gender non-conforming in the default. Do you think there is an age when young people are mature enough to make up their own minds?
I can't speak for others, but for me the concern is about the overall message that we are giving to children and young people.
As a feminist, my views have always been along the lines of: Males and females are equal, and your sex should not affect your life choices in any way. Everyone should be able to choose their own style, hobbies, friends, job, living conditions etc without being restricted by their sex. Sex is only relevant where the physical differences between males and females are important, or where different treatment is necessary in order to address historical discrimination. So, in circumstances such as hospitals, prisons, sport, shared spaces where women may be vulnerable. Also women's campaign groups, such as abortion rights, that relate directly to our biology.
Trans rights activists (and the gender ideology that is being taught in schools and through both mainstream and social media) seem to me to have a very different message.
Their message seems to be along the lines of: Gender is of utmost importance to individuals and the way they live their life. Everyone has a gender identity, and for most people this identity 'aligns' with their biological sex, so that their sex feels right in some way. If you do not experience this internal sense of an identity that aligns with your sex, you are 'trans'. This means that you are part of a vulnerable group who have a high incidence of self-harm and suicide. It may also mean that your body does not match your internal gender identity. If this is the case, this will cause you all sorts of problems. You should therefore be allowed to change your body permanently in order to make it match your internal gender identity as closely as possible. In this way, your biological sex will be changed (or your true sex will be revealed).
My problem, my concern, is with this trans narrative. It is simply not true that everyone experiences an internal sense of gender. It is incredibly damaging to suggest that children and young people should be able to label their gender identity and decide if they are 'really' men, women, both or neither.