It's about the power dynamics. I've slept in a lot of mixed dorms in mountain huts. This means that I typically have a friend present (whoever I'm climbing with), and there is hopefully the bystander effect - a whole load of other people around if anyone tried anything funny.
Being alone in a lockable compartment with a biological male I've never met before and don't know from Adam - totally different.
As for how many men and how many transwomen are predatory and the precise percentages relative to the population at large - well, it doesn't matter. There's behaviour well short of sexual violence that worries me. I'm sure there are very nice transwomen out there, but how would I guarantee I got one of those rather than the sort who tweet about "punching terfs", "starting a ruck at Milwall", the sort who verbally attacked the woman on the picket line the other day, the guy who allegedly punched the woman at speakers' corner (I have seen the video, but don't want to prejudice any court proceedings one way or the other). There are plenty of instances of transactivists showing very male-pattern aggressive and violent behaviour towards women. I would not want to be in a confined space with any of them.