A CONSCIENCE vote on same-sex couples adopting children was needed in a bid to avert a serious threat to marriage, Tasmanian Catholic leaders said yesterday.

Archbishop Adrian Doyle and senior officials said the importance of a free vote on gay adoption was akin to that on abortion.

Centacare founder Father Clem Kilby and adoption social worker Sister Philippa Chapman argued the proposed law change would put the rights of adopting parents ahead of children and relinquishing parents.

Sister Philippa said: "Any adoption practice which is so applicant-centred that it would place a child to meet the needs of parents is reprehensible.

"Such practice would use human beings, non-consenting, to meet the immediate demands of others. The human being is rendered a chattel."

There are only about 20 adoptions a year in Tasmania and most adoptions by same-sex couples would occur in a situation where the child from a previous partnership was already living with the new couple.

The Tasmanian Gay and Lesbian Rights Group slammed the church as disregarding the welfare of children and the feelings of lesbian and gay Tasmanians and their children.

Group spokesman Rodney Croome also said the Catholic leaders were not thinking of foster children, who are already cared for by same-sex couples in Tasmania.

"The thinking in welfare is that adoption of foster children is ideal where appropriate," Mr Croome said.

"Adoption reform is not about finding children for gay couples, it's about finding supporting and stable family and home environments for children," Mr Croome said.

Archbishop Doyle has based his submission to a Tasmanian Law Reform Institute report on legal questions, while Father Kilby and Sister Philippa have referred to their experience with the largest non-government adoption agency.

Father Kilby said each politician had their own background and convictions and these would be violated by voting on party lines if they did not agree.