Long answer:
If we were to read the first few chapters of Genesis exactly as written, with no outside influences, the plain reading says the world was created in six days, mankind was created as Adam and Eve, and they sinned putting all their descendants (mankind) under a curse.
However the account is brief. It doesn't say a lot of things - it doesn't say how many children Adam and Eve had, doesn't specifically say who they married, what skin colour they had, what crops Cain was growing... So people speculate about these matters. Which is understandable. But WHY do people speculate that there were other people besides Adam and Eve?
The fundamental reason for this, for many people (I can't speak for the posters here) is because they already believe that science has shown a long history of human evolution etc, and they realise that this does not agree with the Creation account of Adam being created from the dust of the ground. They are faced with three options: Reject human evolution, reject Genesis, or fit evolution into Genesis somehow. They don't want to reject evolution, as they have a strong faith in science. They don't want to reject Genesis, as it is so foundational to all aspects of Christian theology. So they try and blend the two.
This blending takes a number of different forms - for instance the "gap" theory which sticks millions of years of history in between Genesis 1:1 and 1:2, and the "day-age" theory which says all the "days" of creation were long periods of time. Common to each of these is the existance of pre-Adamic humans, or human-like beings, who evolved and created the fossil record of human evolution. The idea that Cain married one of these is speculation that follows the acceptance of the existance of these people. The key issue here is to look at whether these people needed to exist at all.
If you have a good look at what bones have actually been found for these so-called "cave men", the lack of evidence becomes rather shocking. Most skeletons have been formed from one or two bones of disputable origin. The actual fossils in existance are just as readily (or more readily) interpreted as humans, diseased humans (Neanderthal looks like a modern human with severe rickets, and the fossils were initially classed as that before evolutionary theory was read into them), and apes. I would highly recommend the book "Bones of contention" by Marvin Lubenow if you want to understand this further, I used that as an "alternative textbook" throughout my final high school year of biology (human evolution was the key topic of the year), and it made such an impression on both the class and teacher that I had a fellow student track me down years later to see if I still had it. I should buy another copy.
http://www.answersingenesis.org/store/product/bones-of-contention/
As a scientist, I reject evolution entirely as it defies basic biology. Natural selection is a very real and important part of biology, as is artificial selection. Mutations are real as well. But when you add them all together you don't get "onward and upward" progress to new creatures, or new features that didn't exist previously. You rather get selection towards more and more narrowly specialised creatures that are right for particular niches. Organisms with more heterozygous gene pools (containing lots of options, like wild mountain sheep) are bred to get specialised organisms with homozygous gene pools (e.g. highly bred stud fine-wool Merino ewes), that are excellent for one specific purpose (the production of fine wool) but can never be bred back to the original without cross-breeding as they have lost a lot of the genetic information preset in their ancestors.
Real-world biology and breeding is the loss of genetic information to select the bits that are most desired. Evolution requires faith that over long periods of time, the processes we see today are actually wrong, and you actually get an increase in information through mutations (which we see damaging information, not creating it) and natural selection.
If I believed there was no God I would be forced to have faith in evolution as the most viable atheistic option, despite its inherent flaws. However as I know God is real there is no reason to shut my eyes and blindly accept something that defies so blatantly the science I work with every day, I know rather that God created the world. He created a load of organisms containing a lot of information, and these have been selected and specialised over time, which fits both the Creation account and real, present-day science.
If there is no evolution, there is no need for ape-men. There is no need to speculate that there might have been humans before Adam, we can simply accept the creation account in the most natural reading of the plain text.
This does lead to the conclusion that both Cain and Seth must have married their sisters. But this is not a problem biologically (can elaborate if you like), and if we think it is "icky" we have to remember that lots of other people think polygamy is "icky" so that's no reason to think it isn't true - there was no law against it at the time. The Bible is clear that Abraham also married his sister (half-sister), so I don't see a problem with accepting this - unless you have a larger presupposition you are trying to fit into the text as well.
