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 s***n, then it is Makruh (blameworthy), but there is no decisive evidence (to forbid it).
These parts are not dirty like a**s, but it is ordinarily disgusting to man. But there is no decisive evidence to make it unlawful, especially if the wife agrees with it or achieves o****m by practicing it. Allah, Exalted and Glorified be He, says: "And who guard their modesty, save from their wives or the slaves, that I heir right hands possess, for then they are not blameworthy, but Who so craveth beyond that, such are 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 s*x, for that is strictly forbidden.
Dr. Ali Jum`ah, professor of the Principles of Islamic Jurisprudence at Al-Azhar Univ., says further that l*****g, sucking and kissing spouse's sexual organs are all allowed, as long as it gives a person sexual gratification that will keep him away from Haram (unlawful) or starring at opposite s*x (marriageable). But every Muslim must keep in mind that sexual intercourse is just a lust and passion that must be satisfied in a lawful way; it's not to be perceived as necessary as foods and drinks, as it's the case in the Western perverted ideology.
Shedding more light on this, Dr. Sabri `Abdul Ra’ouf, professor of Islamic Comparative Jurisprudence, at Al-Azhar Univ, says that it's allowed for the married couple to enjoy each other as long as what they do does not run counter to the teachings of Islam or violate the public norms. He also gives support to the view of Sheikh Al-Qaradawi that oral s*x or kissing private parts of the spouse is something viewed disgusting to Muslims, but if the aim is just kissing (without having constant indulgence in it) it's not sinful to do that, but people of high morality normally keep away from that, as not to give in to imitating non-Muslims.”
In conclusion, it has become clear now that oral s*x is not prohibited, but it is not the normal choice for committed Muslims and Muslimahs. That's, despite that oral s*x is not Haram, it is completely disgusting and does not conform to the pure taste and decency of a Muslim personality.
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 s*x or such practices cause mouth cancer or form a danger on the health of a person who practices it, then it becomes totally prohibited.