I find this a difficult issue to understand perhaps all the more so because I don't think my instinctive view fits neatly into either that of trans activists or those they would identify as TERFs.
I am supportive of people who identify with a different gender from their biological sex in doing so. I believe they should be treated with respect and not subject to abuse or harassment. I appreciate that they likely have challenges in many areas of life including access to appropriate and sensitive healthcare.
What I find harder to understand is the political argument that their original biological sex (whether male, female or intersex) did/does not exist, and that by extension biological sex generally does not exist, and that anyone who refers to it is attacking trans people.
FGM is a violation against the female sex. It is carried out against people who are born girls with female genitalia whatever their gender identity. If a man requires an abortion, it is not because he is a man or completely unrelated to biological sex. It is because he was born with female reproductive organs. He is in this respect still subject to the concerns of his original biological sex even if socially he identifies as a man.
It seems very odd to suggest that this biological sex does not exist and that it is just chance whether the person affected by pregnancy or FGM is a man or woman. Feminists have worked hard over the years to protect the rights of those affected by being born female. Some of those rights are inseparable from biological sex because they only affect those born female (whether they are women or trans men) and some of those rights affect anyone who is socially identifiable as female (whether they cis women or trans women). In addition trans people also suffer from transphobia and rights specific to the discrepancy between biological sex and their presented sex.
It seems to be missing the point to berate feminists for not distinguishing clearly between the two kinds of oppression they are fighting (ones directly associated with the biological differences of being born female, and ones associated in appearing female - and probably have their origin in the biological differences of being born female, even if the individual suffering them was not) and lumping them under a header 'things which oppress women' because they also oppress either trans women or trans men too.
Feminists are not responsible for this oppression, the patriarchy is. It is the patriarchy that wants to deny cis women and trans men access to abortions because it gives power to those born with female reproductive organs. It is the patriarchy not feminism that socially excludes cis women and trans women from positions of power so they can be occupied by cis men.
I'm not trying to minimise the additional, intersectional oppression trans people face, but I don't think that feminists are the prime cause of this additional oppression. They/we are an easy target because we care about equality and can be hurt by accusations of excluding people due to isms, so we engage in debates that those who are not feminists do not.
I feel I may be likely to annoy both sides with this post because of not fitting neatly into a particular type of feminism box. I know I don't have enough knowledge of the different ideological arguments at work here. Sorry! Apologies for being clunky with terms like cis and trans, its hard to be specific without offending people, I don't agree with people saying it's 'cissexist' to talk about things which affect those born female as women's issues.