How did adam and eve figure out the sex thing?

So if Cain and Abel were their sons, one of Adam's and one of satan's, did it not say some where in most translations that they went out and took wives?
Where did the wives come from?
If Adam and Eve were the first humans where did the wives come from?
Did they interbred with Eve?
How did they multiply?

I have often wondered that too! Only other option would be a sibling. Unless God made more men and women and the bible (lots of parts left out/ mistranslated) didn't mention.

some religions preach there were more than one Adam and Eve, just this set that we hear about ate of the forbidden fruit. One comment said sex with the serpent was the "forbidden fruit" hence why they were kicked out of the garden. The serpent, according to a teaching was half human- i.e reptilian. Hence inbreeding and development of the reptilian race? not sure....