Firstly, the exclusion being talked about is exclusion from legal rights in relationships because of gender. It is not an argument to remove 'any exclusivity' to anything - that is a straw man.
Secondly, we all use analogies to try and see commonalities in issues so we apply common principles. Marriage and slavery are not the same thing but can, as analogies, illustrate how society has changed its attitudes over time (as can universal sufferage, child labour laws, indigenous recognition, separation of church and state etc are all things that can be used to compare how society values have changed and to illustrate common ways to approach new issues). There needs to be care to make sure we don't push analogies too far or apply them inappropriately but I can see why looking at them helps build a better idea of how to approach issues.
Thirdly, I can sympathise with people who hold deeply held convictions about social issues that differ from their employers and feel they are being forced to support a social issue they disagree with.
This thread has two things going on and that clouds the arguments.
a. what happens when an employee has a moral objection to a social issue that their employer is promoting?
b. is SSM something that should be recognised in Oz?
(for me question a is answered: really not sure and b: yes)