Sheikh Yusuf Al-Qaradawi tackling this issue stated that: “Muslim jurists are of the opinion that it is lawful for the husband to perform cunnilingus on his wife, or a wife to perform the similar act for her husband (fellatio) and there is no wrong in doing so. But if sucking leads to releasing semen, then it is Makruh (blameworthy), but there is no decisive evidence (to forbid it).
These parts are not dirty like anus, but they are usually disgusting to men. But there is no decisive evidence to make it unlawful, especially if the wife agrees with it or reaches orgasm by practicing it. Allah, Exalted and Glorified be He, says: “And those who safeguard their private parts, save from their wives or the female slaves, that their right hands possess, for then they are not blameworthy, but Who so craves beyond that, such are the transgressors.” (Al-Mu’minun: 5-7)”
In the light of this, scholars maintain that the husband is allowed to enjoy his wife through any means of enjoyment except anal sex, which is strictly forbidden.”

Here, it should be noted that one of the main objectives of shari`ah is to safeguard the life of people and keep them healthy. Based on this, if it is scientifically proven that oral sex or such practices cause mouth cancer or form a danger on the health of a person who practices it, then it becomes totally prohibited.