The Book of Enoch includes speculation of angles outside the garden interbreeding with AE’s lineage. If interbreeding was possible, that makes them essentially biological humans from a scientific point of view. Yes, they did not mean the same thing by that term, but it demonstrates the idea of input into AE’s lineage was not a theological problem.
There is a lot of evidence that the Book of Enoch was widely read at the time. I know that a lot of scholars identify “Son of Man” with Daniel, but it also makes sense as a reference to the Book of Enoch, where that title appears all over the place as a clear reference to the Messiah. The fact this was a title that Jesus used indicates, not that Enoch was necessarily inspired, but it does reinforce the idea that it was part of the cultural stream.
Paul, it seems, almost certainly would have known of Enoch. If mixing with AE’s lineage was a problem, it seems that Paul almost certainly had enough context to have had cognitive capacity to speak against it, but he did not.
Add to that the historical speculation about Cain. See some information here: Daniel Deen and Joel Oesch: The Lutheran Voice and Crosswise Institute.