Male, female, other, and leave blank / prefer not to disclose are all the options I would list. It's important to allow women and men who do not want to disclose their gender that option. It's also important to allow people who would like to list their gender as other to do so. Gender is an extremely tricky issue. Whatever you do, I recommend NOT listing "male to female" or anything of that sort. Transgendered people identify, from my understanding, as their adopted (actual) gender, not the one society would often assume them to have. I hope this helps.