It's the pronouns, and what people assume comes with them.
No employer or potential fellow colleague is going to mind if you're a short haired female called Bert, who dresses in men's suits, plays Dungeons and Dragons, and resists the assumption that you ought to make the tea.
But many employers and potential fellow colleagues are going to mind if you indicate that you would like to dictate the words people use to describe you when you're not there.
Some will conclude that your wish to control the speech of others indicates one or more of the following: you're not a team player; you will seek recompense if people slip up or decline to play along, either through HR or the police; you are not outward looking; you will focus more on your identity than the work.
Some will worry that you harbour authoritarian, homophobic and misogynist attitudes, because of your stated belief in gender ideology.
Some employers may have a workforce made up primarily of white middle class 20 somethings who put pronouns in their emails and here you might be a great fit. Many employers, especially in less than elite jobs, will have a far more diverse workforce and will worrying about how to balance your demands against their other employees' needs.
There's also the fact that many jobs come with customers, service users, clients or children. Again, your wish to control the speech of others with have an impact here too. Some of these stakeholders may be neurodiverse, speak English as an additional language or be otherwise disadvantaged. The employer might suspect you will proselytise about your belief system and feel that is not appropriate for their service users, especially if children.
I would be strongly suggesting my child let go of the requirement for preferred pronouns. They will find a new kind of far more genuine validation when they are valued for the work they do when they get a job. It will do them good to spend time with people from different walks of life and to discover that we all have far more in common than these divisive belief systems would have us realise.