I am confused by the debate because it seems obvious to me that Liberals are not moral relativists and the Catholic church is not morally absolutist.
Liberals are not moral relativists. Proof? If we were relativists we would not and could not criticise the Catholic church on moral grounds, as in relativistic terms its moral assertions are self-legitimised.
The Catholic church is not absolutist. Proof? The moral precepts of the catholic church have changed more radically than the precepts of liberal humanism in the past 500 years.
These seem to be such obvious counter-arguments that I can't understand how the debate has been given a moment's attention by anyone.
Incidentally people often jump to the next question - how, then, do we know what we ought to do, and what social rules ought to be followed? My answer is that we do it neither by appealing to an existing social system, nor to an absolute external authority, but by reference to the future. We don't know what to do, but we make a best effort to bring about the future that we want to exist: 'The prophetic soul of the wide world dreaming on things to come'