As far as Islamic Shari`ah is concerned, it is haram for the person who is junub (in a state of impurity following sexual activity) to perform salah (ritual Prayer), perform Tawaf, stay in the mosque, recite or touch the Qur’an. Everything else including eating and drinking of course is permissible.
The eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states: “In the state of janabah (ritual impurity), of course one is permitted to eat and drink. It should be quite clear that the janabah has no effect except in Prayer and the recitation of the Qur’an or entering the mosque according to some scholars.

Therefore, before taking ghusl, the junub is permitted to eat and drink and there is nothing in the Islamic shari`ah to prevent him/her from doing that.