Maternity rights are female rights. Gender identity of said female is irrelevant to their claim on said rights
I wonder whether the actual solution in the end is to stop using the convenience of 'mens' or 'womens' rights, and be more specific about the criteria which apply in each individual situation.
I'm not suggesting losing the word women to apply to our sex category when campaigning - but when we get down to formal wording, going into more detail.
Eg
Employees who have given birth are entitled to xyz from the date of the child's birth
Employees who acquire parental resposibility for a child during their employment (whether through birth or adoption) are entitled to xyz
I think it might be what people are trying to do with 'birthing parent' (being generous) but language is key to avoid making us feel like body parts 
Also
Only swimmers who have not been through male puberty may compete in the women's category (FINA's actual decision - beautifully put)
Only students who are registered with the school as female may use the girls toilets and changing room (schools always see identity documents when students register, and GRC is only after 18, so that would be sufficient. IF schools were on-side)
What we can't have is keeping the existing broad-brush men/women categories, but change those categories to be based on