Well quite. Which is why I find this thread a bit off really. I don't see why people are so eager to state that the perpetrator is a transman. You've misunderstood the OP... as have many others.
The eagerness is just continued questioning of the Stonewall ethos and its 'Trans Umbrella' - that's Stonewall's own name for their re-defining the meaning or Trans, under which every single human being can be included - go have a look!
Such responses are partly irony, lots of sarcasm and a lot of "Fucking hell, here we go again!"
So very many times we are told that a man is a woman because he says so, dresses as such or may have done so on a Tuesday! The BBC and most other media outlets fall over themselves to refer to such men as she, all the time, every time, even when the pictures included show men with beards - we mustn't judge by looks, apparently. And we have to rememebr that beg trans is so hurtful that, whilst not being a MH issue, it is a reason to feel some sorrow for the transwomen who transgress the law!
OP has asked why this is not the case when a woman poses as a man/boy for similarly nefarious purposes. Why is she simply a deceptive sex predator? Why are men who do exactly the same given a new name/label and a possible 'out' for their crimes?
You may not agree with OPs question. But it would be disingenuous to suggest that there is no difference in the reporting of such crimes and that those differenes rely on the sex of a woman and the gender of a man.
OP is aksing why is that?