This could not have been the right approach, because, as most people would agree, the Bible says it's wrong. They would have been tempting them to act against what their scripture teaches (ie apostatize) in order to keep their licences. The best thing would have been either to disqualify all church ministers from acting as registrars, as NS said, or allow them to do so without requiring them to conduct SSMs. The acts of marrying same sex couples and giving practicing homosexuals positions in ministry, both of which a church is currently not required to do, would involve a minister doing something that is against biblical teaching, and thus illogical.

To disallow ministers of religion to act as registrars (for heterosexual couples) would likewise be illogical. This is because the marriage service in (say) a Christian wedding is by nature more 'valid' (probably the wrong word) than a simple civil ceremony, since it is done in the sight of God and includes promises of lifelong faithfulness etc (which civil ceremonies do not require). It thus makes marriages stronger.