A U.N. population conference of 180 nations is running into problems as a small group of conservative Catholic and Muslim developing countries is splitting from the majority on family planning, abortion, and sex education in schools. The conference, a follow-up to the 1994 Cairo population meeting, today is supposed to present an updated action plan for limiting world population growth to the U.N. General Assembly, but disagreements between nations may keep it from meeting the deadline. Money is also a problem: Developing nations committed in Cairo to pay $11.3 billion a year for population programs, but have only paid an average of $7.8 billion annually, and industrialized nations promised to contribute $5.7 billion a year, but have only paid an average of $1.9 billion annually.