I think people generally are too scared to say anything but, in particular within actual LGBT organisations or groups saying anything which even remotely questions the trans agenda would lead to you being thrown out.
Without wanting to get too specific, I know a service user of an LGBT group with a learning disability who asked a question which was not at all attacking trans people but just trying to understand and it was deemed offensive, she was treated really badly and, although she has been allowed back to access services after repeatedly apologising and saying that it was just due to her disability that she hadn't understood or had phrased her question wrongly, she was made to feel really uncomfortable accessing services. If you actually genuinely questioned anything, you would definitely get banned.
As a service user, you have to just decide either to stop accessing LGB-specific services/groups or to go and keep your mouth shut (in lesbian groups which now have a very heavy presence not just of transsexuals, who have always been around but the more modern, domineering 'trans' individuals, including, yes individuals who haven't done as much as shave off their beard but apparently are lesbians now).
Even in my job (in a non-LGBT organisation), I wouldn't be allowed to express anything which questioned the trans agenda so I imagine that the only option for someone working in an LGBT organisation like Stonewall who wanted to protect the rights of LGB people would be to leave their job.