Your analogy doesn't work, though.
For one, niqabs present a obvious problem for security. Veiled women refuse to be identified when receiving public services. Terrorists have been known to flee while wearing niqabs. Cheating on tests has been known to happen through the use of niqabs. The clothing is a bigger issue than what you claim it is, as is evident from the fact that numerous Muslim countries, even conservative ones like Kuwait, have restricted its usage.
Second, abstinence-only education is a belief, not a behavior. In order to be implemented, you need control of the government at some level. You do not need control of the government in order to imprison women through the niqab. The solution to the abstinence-only education issue is not to mandate pre-marital sex, but to have the government support comprehensive sex education. You're once again making your opponent's argument as ridiculous as possible in order to score points here.
Third, you talk as if the only possible threat that can come from fundamentalism is if fundamentalists control the government - that there couldn't be other threats from the fact that fundamentalists have increased autonomy over their own affairs. For example, fundamentalists can set up separate Islamic courts dealing with marriage, women's rights, divorce, child welfare, and whatnot. They do not need majority control, all they need is a government that kowtows to their every whim or else is accused of violating their so-called 'religious freedom'.
On the issue of whether or not the ban will push people away from fundamentalism. Perhaps not, but it will put the state in the way of those who harass women into abiding by this pre-medieval dress code. It will signal to the Salafis that liberal societies will not turn a blind eye to their efforts to Islamify and radicalize the Muslim populations within those societies. It will tell people that they either make an honest effort to integrate into society, or they will be turned away from public services. We'll whether or not people put their fundamentalism before their economic needs.
"How does being allowed to wear a niqab in and of itself threaten anything?"
I already mentioned this. The niqab poses substantial security risks.
http://www.kuwaittimes.net/read_news.php?newsid=MTQwMTY5MzI5Mg==
"And what right does the government have to tell people how they can dress?"
The government is under no obligation to provide public services to those who openly reject the values the government represents. The government has an obligation to protect public safety, to which as I already said, the niqab presents a substantial threat. Governments implement dress requirements for usage of public facilities all the time, and I fail to see why this is any different.
"And finally why are we willing to concede all these points for Christians, but not for Muslims?"
Christians do not have these excessive "requirements" that the state is pressed to accommodate for the most part. Furthermore, while Christian extremism is a problem in America, it really isn't a problem in Europe. I have never ever heard of Christian fundamentalist groups running around in France or anywhere else in Europe. This is a case, again, of false equivalence. Pretending that the problem is the same for all religions, when it is not. I agree that more needs to be done to curb the problem posed by Christian fundamentalism (especially as it relates to their systematic use of terrorism against women's clinics in the US). And if you'll recall, this whole debate started because I said that Christian prayer groups need to be banned from public schools.
As for whether it is true that women who wear the niqab or burqa are fundamentalist, that's like saying that we don't know if Christians who claim that gays should be executed are fundamentalist. The fact is only Salafi teachings of the Hadith interpret it to mandate that any showing of the female body is disallowed. How can this be anything but an extremely conservative interpretation? What on earth could be liberal about the niqab, a garment that's sole purpose is to eliminate a woman's personality and cover her body in a tent because it's a woman's responsibility to defend herself from predatory men?