Well look, the whole clothing thing is, in itself, pretty trivial, but it is one manifestation of the way the prison service takes a much gentler and more therapeutic approach to women (who are seen as vulnerable, damaged and in need of care) and men (who are seen as bad people in need of punishment and discipline).
My contention is that trans women likely to be similar to other women in terms of their psychology, so then need the gentle rather than the tough treatment. You may disagree.
However, it seems to me that treating someone guilty of the same crime more harshly purely because of their chromosomes is morally wrong.