Conservative groups opposed to family planning have turned their fire on the administrator of the Agency for International Development, Peter McPherson. In fact, what they are trying to do is block humanitarian assistance to some of the world's neediest people.

In a meeting last week with Mr. McPherson, the Stanton Group, a coalition of some 40 conservative groups, specifically denounced continued U.S. support for U.N. population aid programs. Following up on complaints to the president by Sen. Jesse Helms, Rep. Jack Kemp and others, the spokesmen for the coalition charged that Mr. McPherson ignored an amendment passed by Congress last fall when he released part of a U.S. grant to the U.N. Fund for Population Activities.

But the legislation in question was specifically drafted, after thorough House debate, to allow continued U.S. grants to the UNFPA. The language only prohibits granting money directly to organizations or countries that support coerced abortion. The UNFPA does not support even voluntary abortions, much less coerced ones, but conservatives are arguing that the organization is tainted because it gives several million dollars to China for other specified purposes. Actually, amendments applying this broad interpretation, which could effectively block all commerce with China, were specifically rejected in Congress. It earmarked $46 million for UNFPA after the debate.

Given this clear directive, AID had no choice but to release the money. It is probably exceeding its discretion in withholding $10 million from UNFPA to signal its disapproval of reported coercive practices in China. But this nicety doesn't slow down the conservatives, who have made it increasingly clear that their real target is any form of family- planning aid. Recently, for example, presidential assistant Faith Whittlesey wrote the president on behalf of "one of the President's key constituencies" to express disapproval of AID policy giving grants to contractors and agencies that promote "un-natural chemical or mechanical meas birth control.

Since U.S. law has long forbidden support for any form of abortion, these "un-natural" methods are nothing but the same forms of birth control used by the great majority of U.S. women of child- bearing age. The groups whose cause is being pressed would deny to some of the world's poorest people the ability to determine the number and spacing of their children. This is a choice essential to improving the health and well-being of families and to reducing resort to abortion and infanticide.

This is cruel. But so is the Stanton Group's attempt to end U.S. aid to starving people in Ethiopia and Mozambique because they are unfortunate enough to have Marxist governments. To its credit, the Reagan administration has thus far rejected these mean-spirited demands.