It's important to respect the freedom of religion, faith schools shouldn't be forced to teach subject matter that is potentially inflammatory in their belief system.
Another example is Darwin's theory of evolution, that shouldn't be compulsory to teach in faith schools either.
Now, obviously I realise the importance of sex Ed as much as the next man, but we can't make it compulsory and impinge on freedom of religion, which is a human right, if we start doing that it's a slippery slope, which human rights are absolute and which ones aren't?