This all comes down to the concept of how we construct reality and how we construct the structures which govern our existance. We've constructed marriage in a certain way and many people would find it difficult to change this and view it differently, This is why a form of "civil union" should be available - similar to a marriage but somewhere between a marriage and a defacto relationship.
If parents can show that they can look after, care for, love and provide for their children then they should be allowed to care for children. I still maintain that in adoption etc that if parents of equal standing apply but one is homosexual and heterosexual should get it simply because it reflects what occurs in reality - a man and a woman conceive a child and the woman gives birth to the child. Other than that, there should not be limitations. This is not because of the "way it's always been", it's because it's something that is true - a sperm fertilises an ova which in turn (over a period) creates a child. Technology has changed this, yes, but the moral and ethical implications of this are still emerging and until we have determined the effects this has on the child the old methods should be acknowledged and used. Even now only a female can give birth to a child.
In society today, legally, there is no discrimination due to sexuality. This is stipulated in both Federal and State legislation. What we do have, however, is discrimination based on the form of relationship. This is what people are afraid of, the form and the nature of the relationship and who it is between. It seems ironic that whilst there is general acceptance of the right to be of a certain sexuality it doesn't extend to the right to be engaged in a relationship due to that sexuality but anyway.
Simply, it shouldn't be marriage because of the connotations of what it is. Creating a new legal institution will not create discrimination. De-facto relationships are accepted in society and are not condemned as being "immoral". These types of relationships, under state laws, have a large degree of protection for both heterosexual and homosexual couples (as under the legislation in NSW they are treated equally).