Here is my reply. It's short but I would like people to add to it.

Donor anonymity means that thousands of people can marry a relative, in violation of the Federal Marriage Act, unknowingly. Clearly this undermines the rights of Australians to marry a non-relative. That is only the start, there is still a moral obligation not to inbreed.
Would you like to enter a loving relationship and marry only to find out later that you are related as half-siblings, or as parent-child? This is unconscionable. This would only be compounded if you had children in the relationship. Who would be held responsible for this mess?
Donor anonymity is not something that only affects people on a state by state basis. This is a national issue and needs to be addressed by a national donor registry. Anonymous donors can travel between the states, meaning that people in different states will be related. The Federal Marriage Act is not a state Act, it is a Commonwealth Act. It affects all of Australia. While donor anonymity exists, it is in violation of this Act.