Why is sexual intimacy between a man and a woman not permissible during Ramadan?

Allah has made Ramadan a season for strengthening faith and piety. Fasting was prescribed for us so that we may attain piety, and Allah Almighty said: "Every deed of the son of Adam is for him, except fasting; it is for Me, and I shall reward for it." The fasting person leaves his food, drink, and desires for the sake of Allah during the day. Fasting disciplines the soul, curbs desires, and trains the Muslim in strong resolve to control his desires, submitting to the commands of his Lord, preferring His beloved commands over the desires and cravings of the body, including sexual desire, which fasting helps control. Fasting instills piety by breaking desires and suppressing whims, deters from indecencies, diminishes worldly pleasures, and breaks the desires of the belly and the private parts, thereby making the Muslim deterred from committing forbidden acts.