Can you get some help with stats from your HR, OP? As pp have said, there are a great deal of issues that effect women only as a result of their biology alone.
Can you model this by way of illustration to show that a separate group is needed that acknowledges the needs and experience of biological females only ? For example take 2 trainees, one male (Jack), one female (Jill) and use your firm's own stats and current equality "performance" to plot their trajectory throughout their 30 - 35 year career at your firm.
Get the stats on your firm in front of you for your next meeting. How many women in partnership, at salary and equity level? How many female heads of practice and in management roles? Get the stats on training contracts awarded to women versus associate, senior associate and partnership roles being awarded over a, say, 10 year trajectory. How does this compare to males?
Biology. A typical female will need to deal with, potentially all or any of the following by dint of her biology:
Workplace sexual harassment;
Being denied work because clients either do not "trust" a female or feel more comfortable with men they can do "bants" with;
Being denied network opportunities, either because these are perceived as male centric activities that a women "wouldn't want to attend" and so are not invited (namely sport) or take place outside working hours when women who are also mothers commonly have childcare responsibilities (can you look at what client schmoozing activities have gone on and who has been invited to attend them?);
Menstruation and life/work limiting gynaecological issue like endometriosis;
Miscarriage;
Fertility issues that disproportionately effect females, in terms of emotional and physical toll - e.g. IVF being required, which will effect the female more even if the fertility issues aren't hers;
Pregnancy, post pregnancy complications and maternity leave;
Childcare responsibilities and carer roles for children with additional needs;
Carer roles for elderly parents - look at the available national stats for which sex is carrying out that burden;
Look at the maternity attrition rates at your firm.
Look at family friendly hours, part time working opportunities etc. Are these readily available?
Look at how many women in your firm go from the fee earning/partnership trajectory to the professional support lawyer roles. How many women versus men end up as PSLs?
Plot Jill's and Jack's likely journey through your firm based on all of the above. Emphasise it's not about sexualities or feelings. It's about biological reality and the way society still expects females to do (and not to do) certain roles and activities. Emphasise that you want your firm to be bucking the national trends of how women are treated - get these colleagues enthused about making your group pioneering in its support of women. But this can't be done if the group needing the help (i.e. biological females) is diluted through unnecessary inclusion (unnecessary because other groups are available) or though the use of off-putting, marginalising terms like "cis" and "identify as".
Oh yes, also go armed with Maya's case - you should not be under threat of being fired for being a GC female, it's a protected characteristic.
Good luck, OP. I know from my own bitter experience that law firms can be the worst.