Because they didn't bear children until after the fall.
Also, in the Book of Jubilees, it states that Adam and Eve lived in the Garden for exactly seven perfect years . . . and I believe it.
"And after the completion of the seven years, which he had completed there, seven years exactly, [8 A.M.] and in the second month, on the seventeenth day (of the month), the serpent came and approached the woman, and the serpent said to the woman, 'Hath God commanded you, saying, Ye shall not eat of every tree of the garden?"
If they had been having plenty o sex . . . there would have been children prior to the fall. Seven years of unprotected sex with perfectly functioning bodies would certainly produce children.
I recommend reading the Book of Jubilees . . . it fits perfectly with the Bible and even expounds upon the vital doctrine of Spiritual Circumcision.