Despite the baser nature and un-natural desire for grossness of the average Christian, Adam and Eve's children had LOTS of people to choose from for spouses. And only an fool would say there were no other people. Sure there were, who else was Cain worried about? Where did all those people in Nod come from, you know the ones he chose a wife from?
Here, I will paste, even though I don't expect anyone to bother reading it here if they don't already know it's in the Bible.
Not incest, it's not even implied. Just wishful thinking from the common folks.
12 When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth.
13 And Cain said unto the LORD, My punishment is greater than I can bear.
14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me.
15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
16 And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.
17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
There is no such thing as a fugitive if there is no society to hide from. There is no need for God to "mark' Cain to protect him from his family, they, of course already know what he looks like and what he did wrong. When Cain leaves he takes nothing and no one, but he laid his wife and they had a bunch of kids together and there were enough other people already to form a city,
Reading is Fundamental. .