Well for starters, I don't believe that this thread is dominated by genuine LGBT parents. I believe (although none of us are in a position to know for sure) that this thread is in the main populated by people who are coming at this from a political angle. We can see from poster user names and history of posting how this plays out, and of course we know Mumsnet attracts GC posters who are keen to influence LGBT family boards.
We can also see by the content of posts that the preoccupation of many is GC concerns, not LGBT family concerns.
The main rationale for splitting the board is GC in nature, and hasn't articulated well how this would help LGBTQ families.
The LGBTQ community are also well aware that splitting the T from the LGBTQ is a well-developed strategy that is being used to undermine us all:
https://www.splcenter.org/hatewatch/2017/10/23/christian-right-tips-fight-transgender-rights-separate-t-lgb
This thread seems to be a point in action; another part of the broader effort that is ongoing to split the T from the LGBT.
Meg Kilgannon from the Family Research Council in the States has given speeches on this strategy. And of course she and the Family Research Council believe homosexuality is unnatural and harmful; gay people are also in the line of sight. But we will be easier to tackle once divided.
The Cass report is clear; childhood experiences and feelings that lead them to assertive a trans identity are complex, and often invoke issues of sexuality. She is clear this complexity has to be acknowledged and addressed in order to provide best support for children and young people and ensure the best pathway forward for them.
LGBT family discussions, therefore, may well include areas of both sexuality and gender identity and Mumsnet boards should encompass and reflect that complexity, rather than attempting to segregate issues that can not always easily be separated. Cass would appear to say this approach would not be helpful.
I don't see a genuine explanation for how a split would support families grappling with these complex areas which frequently come up together in family discussions.