Cardinal Willem Eijk of Utrecht, Netherlands, said a papal encyclical or other magisterial document "might appear to be necessary" to counter the spread of the new theory that gender can be determined by personal choice rather than by biology.
He said even Catholic parents were beginning to accept that their own children can choose their genders partly because "they don't hear anything else."
The church, he said, now had an urgent duty to remind them of the truth of its teaching about the human body.
He told Catholic News Service in a Nov. 7 interview in Oxford that Pope Benedict XVI and Pope Francis have addressed the subject within the past five years as each noted that the theory was taking root in Western societies.
"Perhaps a document only on this problem might be an urgent question," Cardinal Eijk said.
"It (gender theory) is spreading and spreading everywhere in the Western world, and we have to warn people," he said.
"From the point of moral theology, it's clear – you are not allowed to change your sex in this way," he added.
"It is like euthanasia and assisted suicide," Cardinal Eijk continued. "When people first began to discuss them they were unsure," but many people have now become so acquainted with such practices they are now deemed ordinary.
He said many Catholics were now accepting gender theory "in a very easy way, even parents, because they don't hear anything else."
The cardinal's remarks came ahead of the Anscombe Memorial Lecture, which he was scheduled to give in Blackfriars, a Dominican monastery in Oxford, on the theme, "Is Medicine Losing its Way?"
A moral theologian and a former medical doctor who worked at the Amsterdam university hospital before he became a priest, Cardinal Eijk, 63, said he would be addressing the rise of nontherapeutic medical practices, including gender re-assignment and euthanasia and assisted suicide.
He explained that medical advances were driving a culture in which, he said, individualism thrived and gender theory was finding fertile ground.
But he warned CNS that one of the consequences of the changing mores was the emergence of intolerance toward people who did not accept the new ideas.
"We are living in a quite intolerant society," he said. "People are talking about tolerance and they say the individual is free to think what he likes but in practice ... people have to accept this certain view of man, this dualistic view of man and this view of the body as something that is moldable.
"And when you say perhaps that is not a morally good way, you are excluded," he said. "You have to think according to these modern theories or you are excluded – it's (permeating) the university world, parliament, the mass media.
"It is very painful and they will make it for us Christians ever more difficult, I am sure," he said, adding that Catholics must press for the right to live by their consciences if they were not to face harassment or even jail in the future.
Young people, he continued, were a source of hope because he recognized that the minority of those who became active in the church "accepted the whole faith."
"It will be the force of the future," he said. "I think we will be a tiny church, a small fraction of the population at least in the Netherlands, but the Christians who remain will have a life of prayer, a personal relationship with Christ, and they will be clear about the faith and willing to testify to it.
"It will be a tiny church, but a convinced church, and it will be willing to suffer," he added.
Because gender theory is so new, the church seldom denounced it by name until the last five years.
In 2012, Pope Benedict, in an address to the Roman Curia, referred to gender theory when he spoke about the profound falsehoods underpinning an anthropological revolution.
Pope Francis implicitly criticized gender theory in "Laudato Si'," his 2015 papal encyclical on the environment, and condemned "indoctrination of gender theory" as part of a "global war on the family" during a visit to Georgia and Azerbaijan.