The most sensitive, emotional, difficult, and divisive issue at the World Population Conference here hardly appears at all in official documents. Yet it raises deep moral and individual issues, is generating protest and confrontation here, and threatens millions of dollars in United States aid for private-group family planning in 10 countries.
The issue is abortion - whether countries should permit it (or sterilization) and whether, if they do, they should be eligible to receive US government aid for family planning.
Trying to calm the waters here, the executive director of the United Nations Fund for Population Activities, Raphael Salas, says that abortion is only one small part of the broad range of family-planning measures.
Of 85 proposals before the Mexico City conference to slow world population growth, only one refers to abortion, he says.
It urges governments to ''take appropriate steps to help women avoid abortions'' and whenever possible to provide treatment and counseling for women who do resort to them.
Yet the controversy dominates all others here - because of strong moral objections by Roman Catholic and other groups, and because of a change in US policy ordered by the Reagan administration in July.
On opposite sides are:
* Roman Catholic lobbyists and right-wing conservatives who see abortion as murder. No country, they believe, can justify it as a means of controlling population growth.
* Family planners, both government and private, who oppose coercion but who also say that each country has the right to choose its own family-planning methods which, they say, actually reduces abortion rates.
The first group sees a moral imperative in avoiding abortion. The second sees the overriding moral issue as giving parents in third-world countries the right voluntarily to avoid having unwanted children destined to suffer and starve. Largely Catholic groups have come here from the US and Western Europe. They plan a march from the conference hall to the Basilica of Guadeloupe Aug. 12.
A number have registered with the conference as observers. They cannot vote but they can speak briefly from the floor.
Some family-planning advocates acknowledge that some fundamental topics still must be faced.
''The issue of government intervention into private lives is going to be the big one in population policy for the rest of the century,'' says one US official.
''Family planning is a personal, private area. It must be voluntary. But it is needed to give the third world the opportunity to have smaller families if they want to.''
In a message to the conference, Pope John Paul II restated the Catholic Church's opposition to artificial contraception in general and abortion and sterilization in particular.
According to data provided by the United Nations, 18 countries have legalized abortion completely. They include predominantly Catholic Italy, Austria, France, and the Commonwealth of Puerto Rico; five Protestant countries (Denmark, Norway, the Netherlands, Sweden, and the US); two Muslim ones (Tunisia and Turkey); six communist (China, Cuba, East Germany, Vietnam, the Soviet Union, and Yugoslavia); and Singapore.
Another 20 countries permit abortions for ''social and medical reasons'' (such as poverty or ill health). The 20 include seven developing countries (Barbados, the crown colony of Hong Kong, India, Seychelles, Uruguay, Vanuatu, and Zambia), five in Western Europe (Finland, Iceland, Luxembourg, West Germany, and Britain), six communist states (Bulgaria, Czechoslovakia, Hungary, North Korea, Poland, and Romania), and Australia and Japan.
In addition, Argentina, Malaysia, Namibia (South-West Africa), New Zealand, South Africa, Thailand, South Korea, and Zimbabwe permit abortion for maternal health reasons or in rape cases. Mexico allows abortion to save a mother's life or in the event of rape; Bangladesh, to save life; and Ghana and Trinidad for health. St. Kitts and Nevis also allows abortion.
One anti-abortion group, the Right-to-Life Coordinating Committee, has issued a statement in the conference area alleging rich nations are ''exploiting'' poorer ones, and alleging that US aid supports compulsory sterilization and abortion in China by channeling aid through the UN Fund for Population Activities (UNFPA).
The anti-abortion lobby in Washington also cites press reports of enforced sterilizations in El Savador and in Bangladesh with US aid. US officials deny the reports from El Salvador and say they do not condone coerced abortions in China.
Meanwhile, the US policy switch is such a volatile issue that when delegation chief James Buckley, former Republican senator from New York, met reporters for the first time here on Sunday to lay out Reagan's population policies in general , nine out of the 14 questions centered on abortion.
Already barred by law from giving US government aid directly to other governments that permit abortion, the White House now says it will ban aid to private population groups that use money (even non-US funds) to ''perform or actively promote abortion....''
The immediate effect is to threaten $11 million of US aid to the International Planned Parenthood Federation in London. The organization supports small, abortion-related programs in 10 countries where, it says, abortion is legal. The countries are India, Bangladesh, Mexico, Thailand, South Korea, Hong Kong, Barbados, Trinidad and Tobago, St. Kitts and Nevis, and Ghana.
International Planned Parenthood plans to retain current policies until it hears formally from the US about the policy change. It says it has programs in 119 nations, and spends about $60 million a year.
The new White House policy also says UNFPA must certify that it is not using any of its funds for abortion in any country. Otherwise it stands to lose its US contribution, now running at $38 million a year.
Mr. Salas told the Monitor that no country had ever asked UNFPA for funds for abortion programs. All UN funds were given on the condition that family planning was purely voluntary.
Pro-planning groups hope that the Reagan switch is only a temporary move to appease Republican right-wingers in a US presidential election year. If Mr. Reagan wins a final term in November, he may not need his far-right wing so much , the groups speculate. Democrat Walter Mondale favors family planning.
Much now depends on how hard anti-abortion groups such as the American Life Lobby press over the next four months to reduce US family-planning aid.