Yes. He
had to. There was no one else around.
If
everyone came from Adam and Eve and Cain was their son, then he had to have sex
with Adam and Eve’s daughter, Cain’s sister.
The Bible does not mention Adam and Eve having any other children, but
logically, where would she come from?
The only other option would be the daughter of Abel, his niece. This would mean Abel had sex with his sister,
also not mentioned.
While we
may have a problem with that now, God didn’t at the time. Remember incest was
not considered a sin until Leviticus 18, well into the time of Moses.
The DNA
of humans and the purity of their environment did not produce genetic problems
until much later.
Also, it
appeared that man’s intentions of sex became corrupted and men began to look at
their sisters/daughters with impure thoughts. God put a stop to that.
There were no other humans
created except the family line of Adam and Eve, so where else would they go to
repopulate?
The only
intention for Cain to have sex with his sister was procreation and the
preservation of humanity.