This is so frustrating.
If this person did not identify as trans, it would be an open and shut case: a man entering an explicitly women only nude space deliberately is essentially the definition of "intention" to cause offence/arousal. That they were clearly aroused is just extra confirmation.
But because this person claims they are a trans woman, everything is twisted and obfuscated. It becomes offensive to say that they are a man, because it's "transphobic". It becomes a reasonable defence that they had a right to be in that space because they claim a female gender identity, which makes it hard to demonstrate intent. It even allows them to claim discrimination - 'my god, I've been charged with indecent exposure because a transphobic bigot was offended by my existence' - and lobby to be exempted from indecent exposure laws because of their trans status.
I actually think that this is the line the Guardian etc will take.
Yes, there was a naked trans woman in the women's spa. She is allowed to be there because she is a woman. Some Conservative religious people got offended because of their bigotry. This is unacceptable - in the same way that it would be unacceptable for a cis white woman to object to the presence of a black woman in the spa.
There is even evidence that this woman has been similarly persecuted before. This suggests that there is systemic injustice based on white, heteronormative definitions of womanhood, which are being inforced by institutionally corrupt police and cis male legislators. This is literal violenc against trans bodies. Defund the police.
Once you mangle a foundational definition, it becomes possible to argue something superficially plausible and construct edifices of sophistry. It also makes it very hard to argue against, because you are either shut down entirely (TWAW, silence bigot); talking at cross purposes (working with different definitions of the word woman) or left flailing around trying to explain why this particular person behaved inappropriately, how to distinguish that behaviour from the appropriate behaviour for a trans woman in a naked spa, and why the rights of trans women to use women's nude spas (like all other women) should be curtailed because of the actions of one sex offender in one place.
I was rereading this piece by Belstaffie about sexual offending behaviour in men yesterday, which I think is very relevant here:
belstaffieonline.wordpress.com/2020/01/10/trans-identified-males-should-not-be-accessing-womens-single-sex-spaces/amp/
The key point is that certain behaviours by trans women, including deliberately violating women's stated boundaries by entering spaces like toilets, changing rooms (and nude spas!) is highly indicative of predatory sexual behaviour and should be interpreted as such, regardless of any stated gender identity or trans status.