As we receive questions multiple times, we'll try to answer them here for everyone. Here's one example.
Cain must have married a sister because there were no other options. The text is clear that "Adam lived 800 years and had other sons and daughters." (Gen 5:4) Over the 930 years Adam lived, this may have been a large number of people, but they were all still brothers and sisters.
God did not create other human beings. He designed Adam and Eve for the purpose of reproducing and filling the earth with other people. He also declared that "it was not good for man to be alone," so His desire is for men and women to marry and have children. The only options for this were sisters (or later cousins). This is not the result of them being perverts, but the natural progression of the earth being populated by one man and one woman.
Admittedly, this is weird and gross from our perspective. We feel this way is because God declared that it was wrong. It is one of the many things that we feel are wrong because God has revealed that it is (like murder, stealing, lying or adultery). However, He declares it wrong hundreds of years later (Leviticus 18:9) after the earth's population had developed much further. At the time Cain was living, there was no command from God about this.
The other problem we have with it is that we've heard that incestuous relationships produce genetically defective children. That is true now, however, God created Adam genetically perfect. Genetic imperfections come as a result of the fall and develop gradually over long periods of time. They had not yet developed in Cain's life.