The Snail on the Slope by Boris Strugatsky

There is a mysterious dangerous Forest somewhere. A linguist Peretz, having been dreaming of visiting the Forest for years, has finally arrived at the Forest Directorate. He is unable to visit the Forest though, being refused the required permit time and again.
Candide, a Directorate employee, survived a helicopter crash over the Forest several years ago. Saved and cured by the natives, but lost his memory, he now lives among them in a village. With his wife Nava they find out there is more to the Forest people than just villagers, who are in fact a forgotten leftovers from a past way of life, now being discarded by advanced and more powerful parthenogenetic female "comrades-in-arms" civilization based in and around lakes, in full control of the Forest. Sex is considered an atavism by them, an unwelcome relic of the past; a male is regarded as nothing more than a he-goat, and villages are being "overcome" one by one unawares (drowned, together with the males), in a slow but unrelenting action...