The Bible also talks about fornication... I have never taken the teaching to be fruitful to include having sex outside of marriage, have you? I do feel sorry for the kids.
Thou Shalt Not Commit Adultry.. one of the 'big 10', a Commandment. Adultry meaning sex outside of marriage.
I believe that when God said "Be fruitful and multiply" He was speaking to Noah and his sons, who were married.