Why is sexual intercourse prohibited during the daytime of Ramadhaan?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Ramadan is a season of goodness which Allaah has bestowed on His servants in order to enhance their faith, increase their righteousness and deepen their relations with Him. Allaah The Almighty Says (what means): {O you who have believed,  prescribed upon you is fasting as it was  prescribed upon those before you that you may become righteous.} [Quran 2: 183]

It was narrated on the authority of Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Allaah The Almighty says, 'Every act of the son of Aadam is for him, except fasting. It is (exclusively) meant for Me and I (alone) will reward for it…." [Al-Bukhari and Muslim. This is the wording of Al-Bukhari]

The wording of another narration adds: "He has abstained from food and drink, and sexual pleasures for My sake…"

It is known that abstention from food, drink and sexual pleasures occurs only during the daytime of Ramadan, not its nights. There are several benefits behind this, which cannot be achieved except through fasting. Allaah has prescribed fasting to discipline the soul by curbing its desires and preventing it from what it is accustomed to. This contributes to the upbringing of the Muslim individual by training him to have strong determination and real will. This also makes him control the desires and inclinations of his soul without giving in to them. Rather, he is surrendering to the commands of Allaah, avoiding His forbiddances and giving precedence to what is beloved to Allaah over his material desires and lusts. Sex is one of the greatest material desires which a Muslim is required to control. Fasting achieves this goal.

Ar-Raazi said, "Fasting inspires righteousness due to the consequent destruction of lusts and the suppression of desires. It deters ungratefulness and immorality, and decreases the value of worldly lusts. Fasting curbs the lust of the belly and the private parts. So, when one fasts often, these two matters will not control him and satisfying them will be very easy. This prevents him from committing immorality and prohibitions. These are the means for attaining  righteousness." 

Allaah Knows best.