Abbott swipes school on gay marriage

Tony Abbott says his former school is sitting on the fence over its position on same-sex marriage.

Former prime minister Tony Abbott has taken a swipe at his former school after it came out in defence of same-sex marriage.

The rector of Sydney's prestigious Saint Ignatius' College, whose alumni also include Deputy Prime Minister Barnaby Joyce, has written to parents and staff outlining the merits of same-sex marriage.

While stopping short of endorsing a "yes" vote in the upcoming postal survey, Father Ross Jones says many same-sex couples wish to marry for the same reasons as heterosexual couples.

Mr Abbott opposes same-sex marriage but insists his position is not driven by religion but by the fact marriage produces families which are the foundation of society.

"It sounds like they're sitting firmly on the fence, which is a pretty painful place to be,' he told 2GB of his former school on Wednesday.

He also dragged his gay sister, Christine Forster, into the debate, arguing that while she and her partner "do a good job" raising their children, it was best when kids had a mother and a father.

"Chris has made it very clear that as a family we can all get on even though we don't always agree on everything," he said.

In a school newsletter last week. Father Jones said although one strain of the Catholic church's teaching argued all sexual activity including that of heterosexual couples must be procreative, another strain, the school of reason approach, was about understanding what it is to be human.

"Were it not for the school of reason approach, we would still hold that slavery could be justified, or believe that wives were subject to their husbands, contra to what St Paul clearly dictated in the scriptures," he writes.

"Presumably, same sex couples who make such a commitment to each other in good conscience, do so by reflecting on experience and on what it is to be human, using their God-given reason."

The rector of another prestigious Jesuit school, Melbourne's Xavier College, has also defended same-sex marriage arguing the sacrament must evolve with the times.

The schools' position defies that of Melbourne Archbishop Denis Hart, who has urged Catholics to vote against change for the "health and future" of society.