Gender should be opt-in, not opt-out.
(and opting out is extremely difficult and almost impossible in our current society)
I don't think it's right to force people into a system like that from birth. If someone wants to identify as a certain gender later, they can do that. Until they tell you their gender, I think it's best to assume that a person is just a person and not put any gender labels on them based on how they look or act. Even the pressure to choose a label can be stressful, sometimes you're just you and no matter how many labels there are, none of them feel right.
I think there need to be more options for people who don't feel comfortable existing in a gender system.