It’s my theory that the ancient Hebrews and early Christians, like the ancient Greeks, thought that the potential life was entirely in the semen, and that the womb was only a planting place, so that spilling or discarding semen was very much akin to abortion.
It’s also possible to read Onan’s sin as not being the simple spilling of seed, but the robbing his brother of posterity, the sexual use of Tamar without giving her children, the disobedience of God’s law regarding one’s brother’s widow, and his father’s direct command.
Given what happens to Onan, it doesn’t make any sense to me that Judah gets away with having intercourse with a prostitute, the disguised Tamar. What do the Biblical writers think prostitutes do with semen? They block it, kill it, wash it out, they practice abortion, they expose their children.
Orthodox Jewish rabbis permit the IUD and the pill if a couple has serious reasons for wishing to avoid pregnancy, but not the condom, valuing semen over embryos, or so it seems to me. Maybe it’s the physical marriage union that’s more important than embryos.
more information than I need, thank you.