There was a series of books/novellas in the '70s or so, involving a very depopulated Earth due to a chemical/drug error that made all females who lived in modern society infertile. A prophet had earlier started a cult where members withdrew from all modern society, and so those women were still fertile.
The hero and a woman meet on the East Coast, sail to Greece in a yacht preserved by "cocooning," have adventures, are saved by female warriors from Naxos in a helicopter gunship, and generally end up in the Med. The cult also has discovered time travel in the form of portals which open from time to time, by which priests take the message on into the future.
I have googled all sorts of combos of the above and never gotten a proper hit. Any help would be appreciated.