I still don't understand why this is even a topic of discussion. The addition of these fields has absolutely no impact on anyone who doesn't want to use them. If you don't fill it in, it's literally as if it doesn't exist. The answer is because some people have very strong ideological beliefs and they believe their way is the standard by which everyone must live, because they're right, and everyone else is wrong. The argument against this is rooted in an insecure, irrational, and controlling ideology that's in direct opposition to the tenets of freedom, and right of all people to live and express themselves in whichever way they want.