This all smells of the Horse meat scandal all over again.
With the quality of surgery available to anyone who wishes to change themselves in this way, it is really hard if not impossible to tell. In the same way that those Findus Beef Burgers tasted of beef, but where actually horse. I had no problems eating those delicious burgers until it all came out, and I would have no problem fucking some of these "ladies" in the same way. Lets face it, some of them look amazing and we vote with our dicks.
Luckily most are quite obvious though.
Can't help but think the gender situation in western culture isn't helping. As for Adultwork having to allow them to advertise as female, surely the best thing would be different questions. For example change the sex/gender question to something like when born did you have the ability to mother or father a child. Or were you born with either a penis or vagina?
At the end of the day, as already said by many, it is simply a question of honesty and integrity.