I'm quite confused on that issue myself. I also wonder why the daughters are not mentioned in the Bible although everyone agrees that Adam and Eve also had girls.
Someone explained in here a while back that the girls of Adam and Eve were not mentioned in the bible, but I am not really sure why that is. And yes they got married to each other so they can bear children and complete the human race.
Here is a thread I asked a while back...it's close to what you are asking
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Here is a thread I asked a while back...it's close to what you are asking
http://tasbeha.org/content/community/index.php?board=4;action=display;threadid=511;start=msg8366#msg8366