Catholic Archbishop of Sydney Cardinal George Pell has made a submission to the inquiry supporting the traditional definition of marriage.

But two of Sydney's most high-profile Catholic women - Kristina Keneally and Clover Moore - have put in their own submissions disagreeing with their Archbishop.

Christian leaders on both sides of the debate have been urging their parishioners to lobby MPs and senators on the same-sex marriage issue.

Cardinal Pell argued in his submission that marriage was a natural institution where a man and women gave themselves to each other for life in an exclusive relationship that was open to procreation.

''By contrast, although the community formed by a homosexual couple may involve genuine caring, affection and commitment to one another, it is not an inherently procreative community, because their sexual relationship is not designed to create children,'' the Cardinal said.

''Marriage is not simply a loving, committed relationship between two people, but a unique kind of physical and emotional union which is open to the possibility of new life.''

Cardinal Pell said married couples who could not have children for reasons of age or infertility were still married because their sexual union was still designed to give life.

In a submission first published as an article in the magazine Eureka Street, former NSW premier Kristina Keneally said that taking a contrary view to church teaching was not a position she had come to lightly.

''It is formed by prayer, reading, and reflection. It gives me no relish to be at odds with my church,'' she said. ''But it also gives me no joy to see people who are created in God's image unable to fully express their humanity, or live with the rights and dignity that heterosexual people are afforded. I act in good conscience - as a Catholic, I can do nothing else.''

Sydney Lord Mayor and independent state MP Clover Moore said federal parliamentarians needed to legislate for the 21st century.

''Many gay and lesbian couples have loving, caring and and life-lasting relationships. As a society we should value and support those relationships,'' Ms Moore said.

Orthodox Christian bishops, who made a joint submission through the Episcopal Assembly of Oceania, said that they did not wish to use legislation to impose Christian ethics upon people who did not share their beliefs. ''Rather, our concern is that the very institution of marriage, which has a Christian tradition of two entire millennia, should remain as it is, and not be confused with the political debate concerning the legal status of same-sex unions,'' the bishops said.

The Uniting Church's Reverend Eileen Ray used her submission to argue that there was no biblical or other reason why two people who loved each other could not marry.

''Jesus lived at the time when homosexuality was openly practiced by the Roman occupier of Israel, but did not make one statement against the practice,'' she said.