You know, it's funny - before typing that I thought to myself 'didn't I read about one very obscure tribe in the whole world & history which had managed to not believe that men impregnate women?' but after thinking about it for a little while and doing some Google searches, all I could think of was that weird tribe in Patrick Rothfuss's Kvothe fantasy novels who don't believe in 'man-mothers'.

Like Randy, I'm always a little skeptical of these things lest there be another Mead/Samoa incident, and assurances like

That’s right. The islanders do not believe that sex causes pregnancy. They don’t believe in physiological fatherhood. Malinowski was incredibly skeptical about this, so he tried all sorts of ways to see if this was simply a story they told, while they actually the real deal. But no, they assured him that it was really true, that all the white people who insisted otherwise were being silly, that the spirits caused pregnancy, not sex.

Don't necessarily resolve the issue especially since the data was from so long ago. But looking in Wikipedia, I see nothing disagreeing and an interesting mechanism:

Although an understanding of reproduction and modern medicine is widespread in Trobriand Society, their traditional beliefs have been remarkably resilient. The real cause of pregnancy is always a baloma, who is inserted into or enters the body of a woman, and without whose existence a woman could not become pregnant; all babies are made or come into existence (ibubulisi) in Tuma. These tenets form the main stratum of what can be termed popular or universal belief. If you question any man, woman, or even an intelligent child, you will obtain from him or her this information. In the past, many held this traditional belief because the yam, a major food of the island, included chemicals (phytoestrogens and plant sterols) whose effects are contraceptive, so the practical link between sex and pregnancy was not very evident.[2]

So it sounds genuine. Still, one indigenous group out of the many thousands studied demonstrates the point: everyone understands the connection between sex & pregnancy.