The point is that we are creating a more dangerous environment when rapists know they can always get away with being caught in a women's restroom because laws would then protect them and their supposed "right" to be there. Even if a pervert tried to rape a woman, and failed..... it would be simply her word against his. Him being caught in a woman's restroom (even with his pants off) would mean nothing. It, then, would simply be his "right" as a warped gender puzzle nut to be in there with his clothes half-off.
Also, I doubt a sexual criminal will get off (legally speaking) if he was successful in raping or even unsuccessful in raping. For one, DNA evidence would make it difficult to explain how a man got his DNA (bodily fluid or hair) on the accuser/victim. If the rapist was unsuccessful, there is still more than likely going to be some evidence of it. If there was no evidence it would be quite amazing. Not to mention, we are missing the biggest variable here... this has nothing to do with the transgender community as a whole...