Many of the early members of the IPPF and the Margaret Sanger Research Bureau were also members of the (British) Eugenics Society. Margaret Sanger herself was a Life Fellow. She was also a member of the American Eugenics Society.
Julian Huxley, another biologist of unimpeachable darwinian credentials, was an important figure in this sordid history.
Like his grandfather T. H. Huxley, Julian Huxley rose to be one of the most (if not the most) influential evolutionary biologists of his time. He collaborated with H.G. Wells (one of Margaret Sanger' lovers) and J.S. Haldane on The Science of Life, a work which helped fossilized various "icons of evolution" into the public mind until this very day. Julian Huxley was a key figure in the formulation of the "modern synthesis" of darwinism. In fact, he wrote a book called just that: Evolution: The Modern Synthesis (1942). Indeed the very term "modern synthesis" is, I think, due to Julian. Huxley also popularized the works of Teilhard de Chardin (of Piltdown man fame).
From 1931-1962, Julian Huxley served various positions in the Eugenics Society, as a fellow, as a member of the council, as vice-president, and president. He was also on the executive committee of the Euthanasia Society, and was vice president of the Abortion Law Reform Society. Huxley was involved in the creation of UNESCO and was the its first director general from 1946-1948. He also co-founded the World Wide Fund for nature and signed Humanist Manifesto II. As late as the 1960s, Huxley was still openly advocating eugenics: Eugenics in Evolutionary Perspective (Perspectives in Biology and Medicine 6 (2): 155-87, Winter 1963.) Huxley's views on population, birth control, sterilization, euthanasia, etc. are hardly different from Sanger's.
Julian Huxley was a long-time friend of Konrad Lorenz, who, for a while, went a little astray with Nazi "racial hygene" theoretical work. Huxley's archive of correspondence is immense and includes practically all the who's who of the darwinian world: the Darwin family, Gavin de Beer, Theodore Dobzhansky, Richard Goldschmidt, Jane Goodall, Earnst Haeckel, J.B.S. Haldane, Louis Leaky, Ernst Mayr, George Gaylord Simpson, Teilhard de Chardin, H.G. Wells, and Margaret Sanger.
But all that is merely a coincidence.
I'm sure. Just that perfect combination of random events and the right social climate that selected out all other viewpoints and caused them to all think the same way at the same time.