A top Catholic theologian whose views reflect those of Pope John Paul slammed yesterday the approval by a US court of same-sex marriage, saying it sanctioned a "moral disorder against God's creative plan".

"This is a great wound to human dignity that can never be justified," Fr Gino Concetti, an expert on Catholic moral theology, told Reuters.

Fr Concetti, who writes for Vatican newspaper L'Osservatore Romano and has advised the Pope on issues of human sexuality, called the ruling "grave and presumptuous".

The decision could pave the way for the state to become the first in America to legalise same-sex marriage.

The Vatican has not officially reacted to Tuesday's ruling by the Massachusetts Supreme Judicial Court, but Fr Concetti's views would be a foretaste of any coming statement.

"This (homosexual activity) is a moral disorder because it contradicts the natural order, which established a union founded on heterosexual relations," he said.

The Vatican condemned same-sex unions as deviant and a threat to society in July, and has twice urged Catholic lawmakers to vote against bills recognising gay marriages.

In the United States, gay rights activists hailed the court's 4-3 decision as a victory. But some politicians promised to stop gay marriage and some religious groups called the ruling reprehensible.

"This is against the creative plan of God, who made man and woman as two equal but complementary beings not only to love each other but to procreate," Fr Concetti said.

Only one other US state, Vermont, legally recognises same-sex couples and allows them to enter into "civil unions" - giving them many of the legal protections of marriage.