Where did Cain find his wife?

The Bible says that all human beings descended from Adam and Eve
(Genesis 3:20). "If that is so", skeptics ask, "where did Cain find
his wife?"

The Bible mentioned Cain, Abel and Seth as Adam's and Eve's children.
But they also had daughters as well.(Genesis 5:4). Hence, Cain must
have married a sister or niece, who also descended from Adam and Eve.
Since there was only one family then, God couldn't have condemned
intra-family marriage as incest. Later in human history,Abraham
married his half-sister Sarah (Genesis 20:10-12) and Jacob married two
sisters, Rachel and Leah (Genesis 29:16).

As humans continued to reproduce, God prohibited incest. It is now a
sin to sleep with your relative, your relative's former/current sex
partner, or your former/current sex partner's relative. (Leviticus
18:6-18; 1 Corinthians 5:1). Modern science has shown that children
whose parents are closely related are likelier than others to have
serious genetic defects. Also, with so many families from all
countries and races available to pick from, nobody has any excuse to
marry a close relative.

Back to our discussion on Cain's wife, some people point to Genesis
4:16-17, which says Cain fled to the land of Nod, where he "knew his
wife, and she conceived and bore Enoch". "Knew" in this context means
"have sexual intercourse with", not "meet for the first time". Compare
with "Adam knew Eve his wife, and she conceived and bore Cain"
(Genesis 4:1). Eve had been created and presented to Adam earlier @
Genesis 2:21-25, so they were not meeting for the first time.
Obviously, Cain went on self-exile to Nod with his wife and slept with
her there.