Is fornication explicitly forbidden in the Bible? I read the Bible frequently but can't recall a specific instance.

Here are some verses for you (emphases added):

Mark 7:21-23: "For from within, out of the heart of man, come evil thoughts, fornication, theft, murder, adultery, coveting, wickedness, deceit, licentiousness, envy, slander, pride, foolishness. All these evil things come from within, and they defile a man."

Galatians 5:19-21: "Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God."

Ephesians 5:5-6: "Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God."

Colossians 3:5-6: "Put to death therefore what is earthly in you: fornication, impurity, passion, evil desire, and covetousness, which is idolatry. On account of these the wrath of God is coming."