This comment has been languishing in moderation because I've had other things to attend to:
While it's true the fathers could have erred in thinking the semen is the child, what do we make of the Bible doing the same?
"And, so to speak, through Abraham even Levi, who received tithes, paid tithes, for he was still in the loins of his father when Melchizedek met him." Heb 7:9,10
i) By using the introductory disclaimer “so to speak” (Gr.=hos epos eipen), the author tips the reader off to the fact that he doesn’t take this image literally. Instead, it’s a graphic metaphor.
ii) Even if, for the sake of argument, the author of thought of semen in homuncular terms, Levi was the great-grandson of Abraham, not the son of Abraham, so I don’t see in what sense he could have thought of Levi literally subsisting in the loins of his great-grandfather. Is there any reason to suppose ancient Hebrews thought that a man inherited his seminal fluid from his dad, who inherited his seminal fluid from his dad, all the way up the line? Wouldn’t the supply run dry after a few generations?
iii) Although this is a metaphor, it stands for a genuine relationship. Because Abraham was a contracting party to the Abrahamic covenant, he acted on behalf of, and in lieu of, his descendents. They were party to the covenant because their patriarch was party to the covenant.