The last time I looked, not that it has ever really interested me, homosexual acts were legal in Australia*. However, one of the common arguements that sometimes used to opposed civil unions is that if poofs are allowed to be joined together, it will open the flood gates to others like horse rooters to demand to be married. This total over looks the fact that animal rooting is illegal, where as homosexuality is not.
Basically, homosexuals should be suggesting that civil unions are not marriage like and more like a business partnership but in a domestic setting. It should not matter if there is a sexual intercourse or not for such a partnership to exist. This would allow asexual relationships** to be acknowledged as well.
* It has been like this since Tassie made it legal some time in the 90's.
** One example of such a relationship was two widows who lived together but were unable to get benefits for couples because they were not sexually involved with each other.