Via Clive Davis, Dave Hill poses an interesting question:

If I object to the government allowing publicly-funded Catholic adoption agencies to discriminate against same-sex couples (which I would) should I not also object to my local authority allowing Orthodox Jews to have the municipal swimming pool to themselves during Sunday afternoons (which I don't)?

The obvious answer (which some of his commenters give) is 'yes', given that the pool is a municipal pool. But the obvious answer is wrong. The discriminatory act would be making the pool available to everyone, all of the time, given that there are many groups - not just Orthodox Jews, but Muslim women and some women of no religion who are merely modest in their behaviour - who would not be able to utilise the facility for which they have paid their taxes. Allocating a small amount of time to those women is not discriminatory; it is the opposite.

It's a similar argument to that over faith schools. Within the state system, then provided that they stick to certain rules about what they teach - and that is critical - then it's discriminatory not to allow faith-based schools.

UPDATE: About 5 minutes after posting this, I see that Adloyada has said the same thing, only better:

It isn't restricted to orthodox Jews. It's a single sex swimming time. So it offers opportunities to various groups, who also pay for the facilities through their council taxes, who would otherwise be excluded by mixed swimming only setups. These could include strictly observant Muslims, radical feminists and others who feel for a variety of reasons that they cannot be in a mixed sex swimming environment. The majority who enjoy mixed sex facilities get those for the great majority of the time they are open.