While I think /u/sohakira makes a good point about pushing moralizations onto the reader being a bad idea, they didn't account for the kind of story you're trying to write.

You would probably alienate a portion of the audience who cares to know the protagonist intimately. If the protagonist is also the main point of view, then you have to remember that the reader is following them every step of the way.

For myself, I would be okay with a genderless or gender neutral protagonist. However, after too long of a genderless protagonist and realizing that regardless of gender, people have similar experiences in life and ambitious toward similar things, I would get accustomed to their genderless-ness. To suddenly spring a gender on me after so long of not knowing about it would be jarring to me.

To negate this, just like with any well done twist, I suggest hinting at it how important the gender identity is to the nature of the story. If you make readers aware that, for some interesting reason, we aren't told what the protagonist's gender is, that tells us there is a good reason why. We will then read along waiting to find out what it is, and we won't assign a gender in our mind. You are essentially telling us to keep reading for more information, and you are promising us that the reveal will be worth the wait. Does that make sense? It's your typical foreshadowing procedure. That is how I would do it.

If you don't want to mention anything about gender and not foreshadow or say anything at all, then I'm not sure how but there could be a way somehow to do it. It's all in the skill and the execution.