It would not be heresy to say that Cain slept with Eve. (Sick, but not heretical. :-0)
The other two answers you mention are heretical. If you think about it, someone not descended from Adam and Eve did not participate (federally) in the Fall, so you have some theological issues there.
Polygenism was condemned in the encyclical Humani Generis of Pope Pius XII, but of course he was only upholding the received teaching.
ArrogantBustard's #5 responded to me, saying "Lousy answers? That answer is (from a Catholic POV) blatant heresy" . My own #3 stated that the responses quoted from friends and old Sunday school teachers were "lousy answers", esp. given that some came from Sunday school teachers. AB's response was read as saying my answer (singular noun) was heretical, not of the "lousy answers" (plural noun) being heretical.
AB, if I misunderstood your post #5, please ignore my post #24 entirely.