But the move has been criticised by some principals and an anti-bullying program that aims to stamp out homophobia in schools.

Archbishop of Melbourne Denis Hart wrote to about 200 principals at Catholic primary and secondary schools on Monday, urging them to lobby parents to support the current "meaning of marriage".

Archbishop Hart said it was urgent that "the meaning of marriage" and the "implications of redefining marriage" were highlighted.

Principals were asked to give parents a pastoral letter from the Catholic bishops of Australia titled "Don't Mess with Marriage".

"You might also consider asking them to send it to their Members of Parliament with a covering letter," Mr Hart said.

The 15-page booklet said redefining marriage would have "far-reaching consequences". "All marriages would come to be defined by intensity of emotion rather than a union founded on sexual complementarily and potential fertility."

It said it was important for children to have a mother and a father. " 'Messing with marriage', therefore, is also 'messing with kids'."

The Archbishop's intervention came as politicians continued to lobby Prime Minister Tony Abbott to allow a free vote for Liberal MPs on same-sex marriage. On Monday Labor Leader Bill Shorten introduced a bill in federal parliament to legalise same-sex marriage.

Fairfax Media understands that some Catholic principals are offended by the material and have refused to pass it on to students and parents.

The Safe Schools Coalition, which works to create a safe environment for lesbian, gay, bisexual or transgender students, urged principals not to distribute the material.

The coalition's co-ordinator, Roz Ward, said the material could damage the mental health and wellbeing of same-sex attracted young people, by suggesting their relationships were unnatural or not as important.

"At school it is particularly important that they can feel they can be themselves and valued for who they. In a situation where you feel that is not valued, everything else becomes more difficult."

She said same-sex families who sent their children to Catholic schools would see this as a "direct attack on their existence as gay Catholics".