Trump is right on this. When has this been a problem? We already have laws against perverts and child molesters. If a man wants to go into a women's restroom to molest little girls, some law about which bathroom he's allowed to use isn't going to stop him. The argument Cruz seems to be making is that this law prevents him from using "I was feeling like a woman that day" as a legal defense. I don't think that's a legal defense as is. But more to the point, this is something that has never happened to my knowledge, and most trans people just want to use the bathroom where they won't be treated differently.
I never thought I'd say this, but I agree with you completely.