Some people assert that gender has nothing to do with your physical body and is instead an internal feeling, a sense of identity. They believe that if a person has a feminine gender identity, they should be able to use women's spaces regardless of whether they have made any modifications to their physical bodies or not.
These people, I believe, label people who believe that changing physical characteristics should be necessary, 'truscum'. It's their logic I was asking about.
That said, if your own position is that people who have had what used to be called a sex change should be allowed in the facilities of their desired sex (so, no penises in women's spaces) it's a coherently logical one, even if it's not my standpoint, so I appreciate that answer.