The major distinction between fantasy and science fiction is, simply, that science fiction uses one, or a very, very few new postulates, and develops the rigidly consistent logical consequences of these limited postulates. Fantasy makes its rules as it goes along . . . The basic nature of fantasy is “The only rule is, make up a new rule any time you need one!” The basic rule of science fiction is “Set up a basic proposition—then develop its consistent, logical consequences.”
— John W. Campbell (1910–1971), American science fiction writer, editor of Analog Science Fiction and Fact
Many disagree with this distinction. That was written in the 1960s and speculative fiction has come a long way since then.
Today, a genre gender bias is clear.
The Digital Reader explains that SF written by women is more likely to be called fantasy:
In 2013, the judges of the Arthur C. Clarke Award, Britain’s most prestigious science fiction prize, disqualified a number of submitted books on the basis that they were not “technically” science fiction. They were deemed by the judges to be fantasy – a genre that does not require the realism of science – which has twice as many female authors compared to science fiction. As Damien Walters has observed, women’s writing is “dismissed as fantasy, while the fantasies of men are granted some higher status as science fiction”.