There is grave misunderstanding between the freedom for them to choose their lifestyle, and the need (if any) to amend the law to allow that. Under current Australian legislation, homosexual people can choose their lifestyle and opt to unite with their same-sex partners and enjoy certain benefits as a couple. Yet naturally, they are not the same as heterosexual couples and they will never be because of the nature of their union and the outcomes of their union.
whatismarriage.org.au has put together a simple video to show what the Marriage Act is about. See for yourself and decide if the Marriage Act should be preserved the way it is or not.
For more info, go to "Defining Marriage - video of the national webcast" hosted by Australian Christian Lobby (http://www.acl.org.au/).