Strange question! I am sure others can give a host of more sophisticated answers, but how's this, for starters: Sexism is wrong because it treats one group of people who are morally equal to another group of people as if they were not morally equal to that other group.

Why is racism wrong? (Same reason.)

Why is discrimination on the basis of any morally insignificant difference between people wrong? (Same reason.)

But perhaps your real question is a different one: Maybe you are asking why differences in gender are not considered morally significant. To this, I am inclined to challenge you to explain to me why you think they are (or might be), and to regard the idea that they are not as the obvious default. So, here's my challenge: Give me a good reason to think gender difference is morally significant! I deeply doubt that anyone can meet this challenge...but if someone can persuade me otherwise, well, then we can return to the question you asked at that point.