When people accuse women us of being anti-trans, I just give them examples of women working for female-to-male transmen, when everyone else looks the other way and ignores their needs.
For instance...... The government department of Women & Equalities proudly announced a health initiative for LBGT women, specifically mentioning lesbians and transwomen.
It was women who pointed out that, yet again, the female sex was last in the queue. They pointed out that it was more important to the government to validate MTfs than deal with the very real and present health challenges facing transmen.
The LGBT health organisations wouldn't dream of standing up for transmen, if it meant that transwomen's feelings were hurt. In fact, they were silent. They consistently show that they don't care about the female sex. Women stand up for FtMs all the time.
Another example would be the groundswell of concern about teenage transmen. It was places like Mumsnet that cared while the trans lobby used girls on the autism spectrum to further the cause for self i/d. History will judge whether the trans lobby acted in the intrests of teenage transmen. Personally, I think the women's movement may have stopped a catastrophe in the making.
When people accuse feminists/mumsnet/women of being anti-trans, we just point to a record of including transmen in our feminism. We don't care about your gender. It is meaningless to women. Transmen are adult human females, transwomen are not.