If I saw a headline that read something like "Person exposes self in public car park" I would immediately think that the person is male as females rarely commit sex crimes.
True, or at least men don't tend consider things done to them (while asleep for example) as sexual assault in the same way that women are encouraged to, so they tend to go unreported because often the cultural perception of the act is not the same, even though, in actual fact it may be identical in nature. Therefore we can't really know the extent of 'sexual assaults' committed by women, if we are going to view 'flashing' and uninvited touching in a comparable way.
The point is that when there is absolutely no doubt over the biological sex of a perpetrator of a crime, and that person does not identify as anything other than their natal sex it can be reported as 'a man' or 'a woman' and the crime stats won't be fudged either way.
When there is a gender identity issue in the mix and particularly when it is a sex crime, we need to find a way of getting to the heart of whether the perpetrator is in possession of a penis, or not.
The problem with being to 'change gender' or self-indentify with ease as a woman or a man or something else, is that in law it can't be easily disputed. What's to stop EVERY perpetrator of EVERY crime identifying as something else just for the kicks and to mess up the system on purpose?
I don't know how these things work in the criminal justice system but perhaps as an answer to this, everyone should be compelled to disclose a) their biological sex at birth and b) what they identify as now, when being charged with a crime. Then anyone who identifies as something other than their natal sex should automatically go into a third category. So you have Man, Woman, Person. That doesn't mean they are misgendered. We are all persons.
A refusal by the CJS to refer to a transman or transwoman as anything other than 'a person' for the purposes of a criminal trial and compiling sexual crime statistics would go some way towards clarifying the situation.