Brisbane's new Catholic archbishop opposes civil unions, viewing them as a stepping stone to gay marriage.
In his first media conference in Brisbane on Wednesday, Archbishop Mark Coleridge also said the Church would never ordain women.
The 63-year-old will leave his current post as leader of the Canberra and Goulburn Archdioceses in order to officially start as Archbishop of Brisbane on May 11.
Laws allowing civil unions were recently passed in Queensland but the new Liberal National Party government has promised to consider repealing them.
Archbishop Coleridge said the word marriage presumes the involvement of a man and woman and the potential to produce children.
"It's not that the Catholic Church finds gay marriage unacceptable, we simply find it impossible," he said.
"Don't call it marriage. If you do call it marriage you run the risk of degrading marriage by reducing it to any one of equally valid or invalid options."
When asked if civil unions were an acceptable alternative he said they were not as "problematic" as same-sex marriage but raised their own issues.
"Experience would suggest ... that civil unions are a way towards homosexual marriage," he said.
"I am uneasy about civil unions for that reason."
The archbishop doesn't believe the church will ever have women priests but said in future, male priests may be allowed to marry.
"You can't have the Catholic church without priests and at this point of the church's life that means male, celibate priests," he said.
"The male bit I think, is unnegotiable. The celibate bit is negotiable but not at this time. There's nothing about the Catholic priesthood that says the priest has to be celibate."
Archbishop Coleridge said he's keen to meet with Premier Campbell Newman to express the needs of his parish.