Nicholas Tonti-Filippini writes in the Sydney Morning Herald:
In an extraordinary show of unity, more than 50 national leaders of Christian churches have endorsed a document on marriage as a legal institution that promotes and protects the identity of children and their internationally recognised right to know, to have access to and to be nurtured by both their mother and father.
... A child's relationship to both mother and father is inherent to marriage. Children conceived by other means may find themselves with people in parental roles who are in a same-sex relationship, but such relationships are not the origin of the child. It is likely for children to be loved and nurtured in such a household, but however good that nurturing, it will not provide the biological link and security of identity and relationship that marriage naturally demands and confirms.
If marriage were redefined, the law would teach that marriage is fundamentally about adults' emotional unions, about romance only, not complementary bodily union or generating and nurturing children.
What is at stake is an ideal that seeks to ensure that a child has both a mother and a father. That the ideal sometimes breaks down or that there are exceptions to it does not make marriage any less ideal. The bodily union of mutual love that is integral to marriage helps to create stable and harmonious conditions suitable for children, and the children can look back to an origin in the love of their parents.