A gay Catholic activist and author says the church should get its own house in order instead of throwing resources at an anti-gay marriage campaign.

Victoria's Catholic bishops are trying to mobilise their parishioners against the two bills before Federal Parliament that would change the definition of marriage.

The church will circulate 80,000 letters urging Catholics to make their views known on the gay marriage debate.

The Bishop of Sale, Christopher Prowse, says marriage is a sacred institution that government should not tinker with.

"This has served us well for a basis of security and happiness in society," he said.

"We do feel that the bills before Federal Parliament as the year goes on seek to redefine marriage as we know it and this is a very serious matter for us."

But his argument has left some gay Catholics unimpressed, including Michael Bernard Kelly.

Mr Kelly is the author of a book about the gay experience within the Christian faith and was the spokesman for the gay Catholic activist group Rainbow Sash.

He says Victoria's bishops are out of step with most of the Australian population and many Catholics.

"The Catholic hierarchy also has a lot of housekeeping of its own to do in terms of dealing with issues of sexuality and power before it starts trying to tell gay people or society as a whole what it should be doing," he said.

The bishops say their position implies no lack of respect for people who identify as gay or lesbian, but Mr Kelly says that is disingenuous.

"I think to say on one hand that they respect and love gay and lesbian people, and on the other hand that our relationships potentially damage the institution of marriage and damage society, is really quite offensive," he said.

And he has described as "limited" the letter's assertion that gay marriage is not possible because it separates marriage from its grounding in biology.

"I think it is a very limited view. I mean marriage clearly means a lot of things in contemporary society," he said.

"The church itself accepts that couples who are infertile or who are very elderly can still be married and have loving sex.

"If that is the case, why can't two women who love one another marry have loving sex?

"This idea that somehow because this bit fits there in a man and a woman, that somehow that is what should determine how society constructs its legal recognition of marriage, I think is almost infantile.

"We have to look beyond that to the quality of relationships."

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Mr Kelly says he accepts that all religions have the right to offer sacraments and rituals on their own terms.

"I think there is no question that government should not in any sense be requiring any religious body to conduct any form of religious ritual that they don't believe they should conduct, including same-sex marriage," he said.

"However, there is also plenty of religious groups that do want to conduct same sex marriages.

"Some parts of reform Judaism and certainly many liberal Christian groups want to conduct same sex marriages as a matter of their religious perspective.

"They should have that right too and certainly society and the government should not be requiring churches to offer same-sex marriages.

"But we can certainly have civil marriage and we can certainly allow religious bodies that do want to conduct same-sex marriages to go ahead and conduct them. What's the problem?"