THE MAIN adoption agency for infants in Sydney, Anglicare, has written to state MPs urging them to vote against a bill that would allow same-sex couples to adopt when it is debated in Parliament later this month.

The chief executive of Anglicare, Peter Kell, cites a child's need for both a mother and father among the 11 reasons why same-sex couples should not be given the same rights as heterosexual couples under adoption law.

''Men and women complement each other in their parenting roles as a result of their inherent physical, psychological and emotional attributes. Adoptive children should not be denied this opportunity,'' Mr Kell said.

The Independent MP Clover Moore said her amendments to the Adoption Act would overcome the ''double standards'' that allow gay and lesbian individuals to adopt, but not homosexual couples. Same-sex couples are also used as foster carers by the Department of Community Services.

Ms Moore said in the vast majority of cases, same-sex adoptions will involve step-parenting situations and ''known'' adoptions, where the child already has a relationship with the parent, such as a foster carer.