The Vatican has denounced an American nun after she wrote an award-winning book on sexual ethics, which includes topics such as gay marriage and masturbation.

The Vatican's department responsible for doctrine says the book, written by respected Yale University theologian Sister Margaret Farley, contradicts Catholic teaching and must not be used by Catholic educators.

Published in 2006, Just Love, says gay marriage can reduce hatred, rejection and stigmatisation of gays.

But the Vatican's doctrinal department has issued a long rebuttal to this and other arguments.

The department says "the Church teaches that the respect for homosexual persons cannot lead in any way to approval of homosexual behaviour or to legal recognition of homosexual unions."

Professor of Catholic Systematic Theology at Boston College, Dr Richard Gaillardetz, says the Vatican's criticism is the "fruit of a life of scholarship".

"I should point out that [Sister Farley] is a past president of our Catholic Theological Society of American and a recipient of our most prestigious award, the John Courtney Murray Award," he said.

"So she's highly regarded in the North American Catholic theological community."

Dr Gaillardetz says Sister Farley recognises that many Catholics face an ethical dilemma.

"They're struggling to make a connection between the official teachings and their concrete experiences are spouses, as lovers, as parents," he said.

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"And I think Farley believes that theology has to help make that connection."

Dr Gaillardetz says the Church's leaders fear that essential elements of Catholic identity are being challenged.

He says Sister Farley's writing and the church's response is evidence of activism in the United States involving nuns.

"In the last year and a half, Sister Farley is the second internationally acclaimed theologian to be censured by Church authorities for their theological positions," he said.

Dr Gaillardetz says another nun based at Cordom University also received a notification from the Vatican, indicating that feminist theologians may be under scrutiny.

"The fact that the Vatican has just completed a visitation - which is sort of a euphemism for an investigation of American Catholic religious women - and the fact that they've just offered a statement criticising the Leadership Conference of Women Religious, when you lump all of that together, it gives the impression that there's a major program underway to rein in women's religious communities," he said.

In a statement Sister Farley says her book, "was not intended to be an expression of current official Catholic teaching, nor was it aimed specifically against this teaching".