How so? From a Catholic POV, did Cain's wife fall from the sky, did Cain slept with Eve, or did God make another woman for Cain to procreate with, and is it heresy for someone to believe otherwise?
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.
One of those daughters was Cain's wife. Yes, that would also be his sister ... You know Murphy's Law of Relativity, right? If you go back far enough, we're all related. Brother. Not only are you and I both descended from Adam and Eve ... we're both descended from Noah. How do you think Noah's grandchildren managed to "be fruitful and multiply"? They married their First Cousins. The Biblical prohibition on incest comes much later.
My belief is that the answer to Cain's wife is in Genesis.
1:26.And God said, "Let Us make man in Our image..........
27.So God created man in His own image, in the image of
God created He him; male and female created He them.
31....And the evening and morning were the sixth day.
2:7.And the LORD God formed man of the dust of the ground,
and breathed into his nostrils the breath of life; and
man became a living soul.
22.And the rib, which the LORD God had taken from man,
made He a woman, and brought her unto the man. (notice
she wasn't formed at the same time as Adam, as the
females of the 6th day races, or mankind, were)
Adam and Eve were formed on the 8th day, after God rested on the 7th.
Cain married one of the 6th day creations. Mankind, all races, were created on the "6th day". Adam (eth ha adam - the man Adam)was formed on the "8th day".
Anyway, this is what I see written in Genesis and it has answered one of the questions I have had since I was a child.