No, it should NOT be "women SF writers". "Women X" ("women writers", "women scientists", etc.) is a pet peeve of mine, because it's ungrammatical. You don't pluralize a modifying noun just because the noun it's modifying is plural. If they were girls instead of women, you wouldn't say girls SF writers. You would say girl SF writers. Or female SF writers, which really sounds better than either of them anyway.