One argument I see against raising kids in a "gender-free" environment and truly letting people choose whether to be girl-like or boy-like independent of their sexual anatomy is that males and females are genetically different, and that boys act like boys because they ARE boys, and that girls act like girls because they ARE girls, and people who cross the gender divide are a minority and an abnormality.
We can argue whether gender expectations are social or genetic (i.e., the endless nature vs. nurture debate) - but regardless of the answer to that question (which is currently still up in the air), what I want is not a world without gender, but simply a world that tolerates and accepts people who cross or transcend gender.
What I mean is, if it turns out that, left to their own devices, boys statistically gravitate towards toy guns and action figures, and girls statistically gravitate towards dolls and dresses - that's perfectly okay. But if a girl prefers to play with guns, or a boy prefers to wear dresses - that needs to be okay too.
The problem that a lot of people bring up is that boys are pushed to be aggressive and girls are pushed to be pretty. If a boy wants to be aggressive or a girl wants to be pretty, that's fine (within reasonable limits). What hurts people is when the nonconformist (i.e., the girl who wants to be aggressive or the boy who wants to be pretty) is criticized for not adhering to societal expectations.
It's a method whereby the social expectations are preserved by punishing the transgressor. There's nothing wrong with a boy being boyish or a girl being girlie, so it's ridiculous to consider that part of the problem. The problem comes when we punish the girl who plays in the mud ("that isn't very ladylike") or the boy who paints his fingernails ("what are you, a sissy?"), because that's the difference between saying "boys can be boys and girls can be girls", and saying "boys have to be boys and girls have to be girls," which is implying that "boys can't be girls and girls can't be boys". At that point we're not giving them options, but closing other options off.
To put it in other words, you can ask a boy if he wants to play baseball, or a girl if she wants to practice ballet, and that's fine. You don't have to ask the girl if she wants to play baseball, or the boy if he wants to practice ballet, just as you didn't have to ask the boy about baseball or the girl about ballet. But what you shouldn't do is express dissatisfaction with the girl who wants to play baseball, or the boy who wants to practice ballet.
And, for example, take "the pink aisle" in toy stores, dreaded by mommy feminists all over the blogosphere. I haven't heard of any stores restricting girls to just the pink aisle, or forcing boys to stay out of the pink aisle, so I don't see a problem. The simple fact that there is a pink aisle, or even that many girls and few boys shop there, is not a problem. It's when you tell the girl she has to shop there, or the boy that he can't shop there, that it becomes a problem.
It's important to stimulate diversity and promote an accepting environment for minorities. But that doesn't mean that everybody has to adopt the beliefs of minorities; they just need to provide them with enough space to coexist peacefully in the same world. The error that too many people make is to put more effort into destroying existing constructions than building new foundations.
The fact that there are a lot of girl dolls dressed in frilly clothes, for example, is not a problem. Nor is it a problem if many girls like them. Those girls don't have to force themselves to play with toy soliders just because "girlie girls" are too conservative and not liberal enough. They're allowed to be who they are and like what they like. It may be true that we need more girl dolls in fatigues or more boy dolls in frilly dresses, to provide more options, but that doesn't mean there's anything wrong with there being girl dolls in frilly clothes; nor is it wrong if they turn out to be popular.
I know social customs have a lot of pull on how people identify and express themselves. But what we need is an environment that tells people you can be anything you want (realistically speaking) - whether it's traditional or progressive. Both are okay, so long as neither one presupposes that the other is not. It's like feminists who choose to be housewives. I think there should be women politicians, and women CEOs, just as I think there should be men who are housewives. But if a woman decides she wants to be a housewife, who am I to criticize her for that?