YANBU
I have an active faith, but would not want my son going to a single-faith school. Religious apartheid is only going to store up trouble for later when kids have had no opportunity to encounter different worldviews growing up.
And, of course, it's totally unfair that kids of one faith in certain areas have a choice of many more state secondary schools than kids of other faiths, or none.
I live next door to a state primary school. Literally next door. There is a brick wall dividing us from the school playground. This state school, funded by public money (including mine), refused to even give me an application form as I was not Catholic, and there was "no point". My son from the age of four had to walk 40 minutes every morning to a secular primary school in another borough, and not the three minutes it would have taken to go to the school next door. There is no universe in which this is a sensible way to organise education.