God did not prevent them from taking fruit from the tree; he gave them free will so they could chose not to eat the fruit from the Tree of Knowledge of Good and Evil.
In a similar way, God has given us free will to make choices concerning sexual expression. He has designed it for marriage. But lust, wanting something that we are not entitled to (like sex outside of marriage), and temptation (seeing that so many other people are choosing to have sex outside of marriage), lead some to choose to pursue sex outside of the boundaries of safety that God has created.
What many people forget is that God knows best—He always has and always will. Current research involving family and relationships is also shedding some additional light on this topic. For example, research shows that married couples have more joy than non-married couples, this includes their sex lives.
-- Dr. Simon