Read this great blog posting by the boy's mom:
My son is gay
If you think that me allowing my son to be a female character for Halloween is somehow going to ‘make’ him gay then you are an idiot. Firstly, what a ridiculous concept. Secondly, if my son is gay, OK. I will love him no less. Thirdly, I am not worried that your son will grow up to be an actual ninja so back off.
If my daughter had dressed as Batman, no one would have thought twice about it. No one.