I'm really late to this discussion, but here goes:
I believe God said, "Be fruitful and multiply, fill the earth...."
When I told my boys to "Go out and shovel the snow," their goal was to clear the walks and driveway. When they had accomplished that goal they were no longer expected to keep shoveling. I feel the same about God's command to "fill the earth:" when we have done that, we are no longer expected to continue to the point of overproduction.
From the point of view of Genesis, there was a major reason to multiply and fill the earth. The future of (hu)mankind depended on it. I don't think that is true at this time, and, thus, I do not see the avoidance of pregnancy as a sin. This is not to say that I agree with any form of abortion. To me, this is not the point of a thread on "Contraception," they are not the same.
John Ericksen