There is huge difference between a male prisoner trying to pass as a woman, and a woman prisoner trying to pass as a man. Even if the transman was taking testosterone.
I do think it is problematic to women prisoners if a transman prisoner is abusive to women and/or had some sort of phalloplasty, (especially the kind that can be pumped up by squeezing the fake testicle) and there has been a recent case where a transman abused 2 women and ws convicted of obtaining sex by deception by claiming to be actually male, but we don't know whether they committed the abuse with a neopenis or object (obviously awful FFS)
BUT it is not difficult to segregate those transmen who are a risk to women in women's prisons or monitor more closely those transmen who would seek relationships with other women. And prisoners of this kind who have abused/ targeted women literally actually could be counted on one hand, possibly one finger.
None of the issues with transmen make it any more acceptable for a male born male man to be housed in the the female estate.
Third fourth spaces and/or segregation within single sex prisons is the very simple answer. It is only those who want what is impossible, unfair and unsafe that are making this difficult.
We as a society have already decided we need to deprive these people of their human right to liberty FFS .. because they have committed crimes, and sometimes very terrible crimes, so if that also means other limitations and compromises on their ideal life for the protection of others then so be it.
NO one is suggesting making it worse than it has to be in order to be cruel (even though we may find it hard to have anything resembling compassion for rapists/ murderers/ abusers and are very angry here but we still have our sense of humanity and decency - they clearly don't), but some people's idea of cruel is akin to a child thinking mummy is cruel because they won't let them play with matches. What can you do with that kind of crazy but say well them's the rules? You cannot reason with it, and certainly not by colluding and enabling.
No one is saying they should be kept in total isolation, unless they are seriously dangerous, but if trans only prisons are the most suitable balancing act for safety then that will have to do.
Safety and risk first, then looking at proper rehabilitation programmes.
AND NO MALES IN FEMALE PRISONS EVER.