What most teachers want out of a teaching union is not endless politicking and votes to strike, but protection when needed, for example, in circumstances of vexatious complaints by pupils or parents; or for internal disciplinary procedures and so on.
If you were critical of gender ideology in a union which has come out as pro gender - that is not going to give you much confidence of support when it came to, for example, the isues of pronouns in schools, or keeping pupil gender issues secret from parents, and so on
Also, that poor teacher in West Yorkshire who is apparently still in hiding with his family after he was targeted by Islamists. He was, apparently, thrown under the bus by his union - because they were more concerned with protecting the sensitivities ( over sensitivities) of the Muslim community and the supposed reputation of the school.
Exactly @RebelliousCow . To say nothing of the way Kathleen Stock was thrown to the wolves by her union.