I always find it interesting how these debates always seen to revert back the use of toilets or changing rooms.
The problem is not the persons gender or sex it is about whether they might potentially be a sex offender or not, just because you have transitioned doesn't make you anymore likely to sexually assault someone then someone who hasn't. Similar to thinking that a gay person or Lesbian might always be seeking to take advantage of someone of the same sex, are heterosexuals always trying to do this?
If someone transitions so they they can take sexual advantage of someone else then that is pretty drastic and extreme and I've never heard about anyone who has done this.
If someone is pretending to identify as a different gender in order to abuse someone that is totally different to being trans and is beyond the trans community. Just like people who use the cover of anything else in order to take advantage of someone else, i.e. a teacher, celebrity, priest etc or someone who just pretends to be friendly or helpful in the first instance.