Where Did Cain Get His Wife?

Where did Cain get his wife?  Many contend that Cain married his sister, for Genesis 5:4 affirms that Adam had sons and daughters.  But I’m skeptical of this, for a variety of reasons:

1.  After Cain killed Abel, God gave Eve Seth as a replacement for Abel (Genesis 4:25).  Eve was elated that God gave her other seed!  In my opinion, that makes more sense if Cain and Abel were her only children prior to that point: she was sad that she lost both of her children (Cain left and Abel was killed), and so she viewed God giving her Seth as a consolation.  But why would that be the case, if she had other sons and daughters at that time?

2.  Genesis 5:4 says (in the KJV): “And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters”.  That appears to me to be saying that Adam begot those sons and daughters after he had begotten Seth.  Is not Seth mentioned in this genealogy because he is technically the firstborn, after Cain left the family and Abel was killed?

(UPDATE: As I look at the genealogies in Genesis 10-11, my impression is that genealogies do not necessarily focus on the firstborn.  Rather, they focus on the figure who can get them where they want to go.  Arpachshad was probably not the firstborn of Shem, for other sons of Shem are mentioned before him in Genesis 10.  But he is listed as the son of Shem in Genesis 11 because he's the ancestor of Abraham, and the genealogy wants to arrive at Abraham.  Similarly, I think that the people listed in Genesis 5 are not listed because they are the firstborn, but rather because the genealogy aims to culminate with Noah, and so it mentions his ancestors.)

3.  Genesis 4:16 says that Cain left to go to the land of Nod.  There is no mention of a wife leaving with him.  Then Cain is in the land of Nod in v 17, and he has a wife!

TV preacher Arnold Murray, if I understand him correctly, teaches that Genesis 1 and Genesis 2 describe two separate creations: God created one set of human beings in Genesis 1, and another set in Genesis 2.  In this scenario, Cain went to the land of Nod and took a wife there, and she was one of the offspring of the first creation.  But I have problems with this view for two reasons.  First, Genesis 2:5 says that, prior to Adam, there was no man to till the soil.  Why would this be, when God already created human beings in Genesis 1?  Second, Genesis 5 essentially equates Adam—-the man of Genesis 2 who eventually begot Seth—-with the man who was created in God’s image in Genesis 1.

Perhaps there are responses that can be made to my reasons for my dissatisfaction with the argument that Cain married his sister.  Maybe Eve had a daughter (who married Cain) before Cain left, and she was the only child Eve had at that time apart from Cain and Abel.  When this daughter left with Cain, Eve was bereft of seed.  And Eve did not mention her perhaps because Eve didn’t think that daughters were as important as sons.  Regarding my third reason, perhaps Cain’s wife was not mentioned as departing with Cain because Cain was the head of the family.  Similarly, in the story of the akedah in Genesis 22, we read in v 19 that Abraham came down from the mountain, and there is no reference to Isaac coming down too.