Adam and Eve had something like 500 years (plus or minus 100 years) to have kids. They didn't have TV, the Internet, a huge house to clean or many of the other activities we have today, so they probably had 500 kids. (Assuming single births yearly, but no where does it say they couldn't have multiples nor does it say they waited in between, so it is more likely more than that.)
And, since they were roughly 300 years old when they started having kids, by the time they probably stopped, they could have had 200 year old grandkids, and quite probably some great-grands, great-great-grands etc. The kids didn't lack for choices in spouses.
And, since they were the most genetically perfect people, they weren't passing down defective genes, so there were no reasons why siblings couldn't marry. I come from a family of six kids. Four of us have the same mother and father, but the last two have a different mother. I'm 25 years older than my sister, and even years later my youngest brother was born. I was married before they were born, so didn't live at home to get that full connection most have when they grow up with their siblings. There's a good chance Seth didn't grow up with his wives, nor did Cain, but notice Cain moved away and there were still sisters (or nieces) to choose from. The population growth rate was huge back then given the life expectancy and child-bearing ages. Eve wasn't an old lady when she had Cain. She was quite young for that time.
One more thing, since most people ask this because they want to prove evolution: What's the other choice? Did these people mate with another hominid species? First, I'd rather come from incest than an ape, and second, often interspecies procreations are sterile (like the mule, the liger, or the tion.) Since, even to this day, when we have children with close relatives, the chances of deformities is slim. Adam and Eve were more genetically pure than we are today.