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According to psychologists Cindy Meston and David Buss, women have 237 reasons for having sex. They figured this out by interviewing 1,006 women around the world and wrote about their findings in a book called Why Women Have Sex.
Why has no one asked women this question before? Meston says, "People just assumed the answer was obvious, to feel good. Nobody has really talked about how women can use sex for all sorts of resources." Their main reason was "orgasm, orgasm, orgasm."
Read about their book and the study here. There's no indication if Meston and Buss will be asking men the same question and writing a book. It would probably be too short to publish...