“Well, according to our records, you follow JK Rowling on Twitter, so you’re on your own. Bye now!”
This is the crux for me. Even IF you think the entity that is Mumsnet is transphobic, and/or even IF you think that the entity that is Mumsnet is complicit in society’s transphobia, and even IF you think an individual poster is transphobic (of course all of this is false) it does not mean that posters should be denied the advice of a union. Unions shouldn’t be asking screening questions to an individual’s likes and dislikes, political affiliations etc.
“Have you ever said anything mean about Corbyn?”
“Do you clap for the NHS?”
GMB could quite easily come out and say: “We understand some tiny proportion of Twitter people have concerns about Mumsnet and transphobia, but it’s important to us that we support ALL those in need of employment advice at this time. Using Mumsnet as a conduit to provide essential services to women does not mean we condone or condemn the site itself, but we feel it’s important to put those differences aside if it means more women are supported with employment. Our LGBT members can be assured that this is about the users’ employment needs, not the site itself nor that user’s personal beliefs. We would no sooner abandon them for those reasons than abandon our LGBT members if they were to express a view we did not personally agree with.”
But if they want to only support those who express the purist of views as approved by randoms on Twitter, good luck with that model. 