Not every act of intercourse results in conception, you know.
This is where we need Fred Franz to come back from the grave and speculate on Eve's menstrual cycle for us.
Actually, if you're asking what the real reason is that they didn't have children right away, this touches on one of the most interesting aspects of the first two chapters of Genesis: the two creation accounts depict two different human races being created.
The nameless humans in the story in chapter 1 were created in God's image. Those were the humans who were told to be fruitful.
The nameless humans in the story in chapter 2 were not created in God's image. They were simple-minded, child-like, until they ate from the forbidden tree of knowledge. This version of the human creation story is thought to continue into chapter 3, where they eat of the tree and are then named Adam and Eve because they were now fated to be "dust" and "life-maker".
So the second human race (as encountered in the current form of Genesis; it's actually the earlier, more primitive creation account) was innocent and guileless. It's only after eating the fruit that they became aware of their sexuality. This was knowledge they were not supposed to have.
The first human race was created in God's image, so needless to say, they were not naive and, as intended by God, they knew perfectly well how to make babies.