Yes, but Cain got married before Seth was born, right?
(Gen 5:3 ESV) When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth.
Now, I'm sure you can have a ton of children in 130 years, but did they? I read it and unless I skipped over a verse it doesn't say they had any children before Cain, or any more children before Cain was banished.
*note*
I edited my post because I needed to add the verse to complete my thought.
Well, lets allow the Bible to answer this my brother.
Genesis 5:1-32( ESV )
"This is the book of the generations of Adam. When God created man, he made him in the likeness of God. Male and female he created them, and he blessed them and named them Man when they were created. When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth. The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters. Thus all the days that Adam lived were 930 years, and he died. "