Mantra, when parents enrol their children in schools such as this, there's the expectation that their children will be taught the values and teachings of the church their chosen school belongs to and by staff who are employed to uphold the same religious beliefs as them.
Education is only one component of the Catholic school curriculum, religious study another.
Now these anti-discrimination laws interfere with a schools capacity to employ the required staff to do the job.
The job parents pay them to do.
For example, if I were a Catholic parent and planned to send my children to a Catholic school for both education and religious education, the way of life, culture and everything that goes with it, I would like to think those I am paying to teach my children were people capable of upholding my churches values, and are in fact catholic's themselves, and if it were my understanding they were not, I would question whether or not I should bother paying school fees to send them there to begin with.
The Catholic way is more than just a sermon, it's a way of life, their way of life, why should they be forced to push aside their schools core values to employ those who don't even uphold their school community's beliefs, due to a Green bureaucracy having gone madder?
I was going to say mad, though thought the term 'Madder' more befitting.

It comes down to personal freedoms.... if Gays don't wish to send their children to Catholic schools, then they have the option/choice to send them to a non-denomination private or public school.
Though is this freedom being extended to the schools, and their communities?
The freedom to demand that those they employ to do a job are in fact qualified to teach the job they are employed to do.
Mantra, it's about government control, and over-regulation.