But you're trying to have your cake and eat it too. You're basically saying "People deserve freedom! But not so much freedom that they can do things I don't like!"
Who draws the line, and where?
Basically, I would see school vouchers as being a way to compete between different schools teaching more or less the same things along more or less the same guidelines, and perhaps with no restrictions, on, say, hiring union labor.
I think the state has a duty to provide non-religious education to everyone, even if you're the lone Christian in a heavily Muslim area, say (or vice versa). If everyone could send their kid to a religious school, in some areas, the market would simply not provide for non-religious education.
That line of argument can be extended in all kinds of ridiculous directions. Should we ban Hummers off the roads? After all, they're driving on roads paved with your tax dollars, and you do not approve!
We should also ban rapists from drinking tap water. After all, my tax dollars pay for water treatment, and why should scum of the Earth like them get to drink it?
Do you see what I'm getting at? If you are going to insist that none of your money goes towards things you do not directly approve of, you're going to have to go for a 100% free-market economy - and by that I mean no police, no fire brigade, no public roads...
"Basically, I would see school vouchers as being a way to compete between different schools teaching more or less the same things along more or less the same guidelines"
... except if the schools teach things you disagree with. Like I said, it's "We should have freedom! But only for things I agree with"... which doesn't strike me as very free at all.
As an aside, one of the problems I see with the various interesting people of the internet is that, for a bunch of allegedly smart people, they harbor a lot of hatred and prejudice. You can't surf without running into hate-filled articles bashing the religious, unions, immigrants, poor people, etc ad nauseum.
For example, the law currently disallows polygamy - but if you look at all the cultures around the world you will find that monogamy is somewhat of a Judeo-Christian idea to begin with - at the very least there are many societies where it is perfectly acceptable.
So now the question must be asked: is our desire to have a monogamous society coming from our Christian roots, or is it something that transcends religion and becomes a shared ideology?
There are a great many issues where it's not easy to say "this comes from religion" vs. "this comes from our social order".
The world is certainly a complex place.
State-sponsored teaching that evolution is bunk, and that the koran/bible/FSM is literal truth, though, is probably crossing the boundary, and that's what we were discussing, not monogamy.
The problem though is that, from the government side anyways, the two arguments are not separable. As soon as you propose to limit/expand the freedoms of one special interest group, all of them swarm out of the woodwork.
The problem is also that you cannot say "this is crossing the boundary" without first defining what the boundary is. Sure, teaching creationism (for us) is well beyond this ephemeral concept of "the line", but fair governance requires us to actually define where "the line" is.
The availability and quality of schools is already a key consideration when choosing a neighborhood to live in. Forcing every last kid in Chinatown to go to a school at odds with their traditions is an affront to liberty. Offering the one Jewish family in Hicksville, Alabama the option of (a) moving to a bigger city, (b) commuting to a decent school, (c) homeschooling or (d) putting up with local school they hate the least, is more palatable to me.
In most cases, the solution would be, "don't live in Hicksville, Alabama if you can't stand the schools." But even if you have to, at least you have options (b) and (d). Public schools don't even give you that much.
Seems to me designing laws allowing some to discourage access to theological/scientific/ideological education that others may desire is the unjust approach.
The goal of the establishment clause, of freedom of assembly, speech, and press, is to protect individual ideological freedom from government interference, is it not?
What danger is there to religious or ideological liberty in allowing vouchers to be used at Catholic schools or Muslim schools or schools with Young Democrats cirriculum or schools with an atheist/skeptic focus? That the government would appear to be funding religious or ideological education? It is a danger to appearances only. The government is not paying more for the extra lessons, and the parents have freely chosen the school.
What danger is there to religious or ideological liberty in requiring--through legal or economic means--that children be educated in a school with only politically approved cirriculum? Two dangers: that the unconscious political or religious mores of a powerful majority will find their way into the approved cirriculum and that it fundamentally denies anyone with a minority view the ability to educate their children within their traditions.
It seems to me that the second case is far more destructive to our goal. The idea that we would like to economically prevent most Muslims from educating their children as Muslims ought to be repulsive to a society theoretically committed to religious freedom. Nor is there safety in sticking to a 'neutral' cirriculum; poli sci and civics have to be taught, American History has to be taught, and science has to deal with the evolution/intelligent design thing at some point. Defining neutrality on popularly divisive topics isn't easy -- and supposing the group that agrees with you will retain political control in the long run is foolishly optimistic.
There is the danger--nay, the certainty--that parents given the freedom to educate their children according to their whims will teach them destructive and false ideas. But this is simply an instance of the highest cost of maintaining a free society: tolerating certain evils because the associated liberty is worth it. It is not the job of the government to serve as a dogmatic gatekeeper, rather it is the job of free citizens to persuade one another in the public forums.
It is my opinion that allowing parents to educate their children any how or way they please, without bias for or against any institution except on the basis of its meeting a minimal functional standard, poses no danger to personal liberty and is in fact a great boon.