I would be tempted to say that this is creating a hostile work environment for female employees, who are being unequivocally informed that their protected characteristic of sex is - in the eyes of this company - trumped by the protected characteristic of gender reassignment.
Sex is a protected characteristic under the EqA 2010, and if you raise your objections by stating the above - that this attitude of denying any focus on female people is perceived by you as hostile to women and that how they have handled this by running your complaints past an interest group advocating for two entirely separate protected characteristics is creating a hostile environment for you as female employees - then it is unlawful for any employer to penalise you for this. (This is separate protection in the EqA, that if you raise an issue, whether the issue is upheld or not, it is unlawful to penalise you for this).
However, the usual caveats apply - only do this if you feel you can. If you can't, maybe you could instead explore the option of setting up a group or committee entirely focused on women's issues. Make it female-only (perfectly legal under the EqA and it is unlawful for them to stop you) and next year, if the LGBT committee once again calls the shots on IWD, you could be in a much stronger position to challenge this.
Whatever you do, I would not accept any of the reasons given, even if you just vocalise this to each other for now. If you can't raise this, I would write a statement about this but keep it amongst yourselves for now. Then make it the basis for founding a women's committee/group.
They are actively opposed to centering female people on IWD because of an external debate about women's rights, which the LGBT group perceive as an attack on their community. Which is completely unjustified as that has nothing whatsoever to do with the company's female employees and their right to focus on women for IWD.
At least next time we're talking about female erasure and they call us hysterical, we have all of the attempts to make IWD 2020 all about the men to point at.
Penalizing you because males who identify as trans feminine excluded is completely unacceptable.