British law to restrict preaching

I note that the British Humanist Association is supporting legislation which would make it a crime to preach against homosexuality – if the preaching exceeded certain limits:

Why the British Humanist Association supports Clause 58

There is an extremely high threshold for the new offence - preaching that homosexuality is a sin, criticising homosexual practices or calling somebody names are not covered by this offence. The act must be demonstrated to be both threatening and intended to stir up hatred.

Religious organisations or people with religious convictions who intentionally stir up hatred on grounds of sexual orientation on the basis of their beliefs must not be exempt from this new offence. The right to manifest a religious belief is not absolute, and it is acceptable for that right to be restricted in some circumstances.

Though I'd agree that 'the right to manifest a religious belief' must be restricted in some circumstances, those circumstances don't include supressing speech. I can't imagine how the law could draw a bright line between, say, calling something an abomination and stirring up hatred.

This is a petard with which the British humanists may well find themselves hoist some day.