Shaykh Yūsuf Badāt, Toronto based scholar, teacher and Imam and a member of the Canadian Council of Imams, says oral sex is prohibited in Islam if sexual fluids entered into the mouth and or swallowed.
Answering the question “Is oral sex allowed in Islam? Can a woman or man kiss or suck each other’s private parts?” Shaykh Yūsuf Badāt wrote:
Jazāk Allāh Khayr/ Thank you for your question.
Oral sex refers to sexual contact between the mouth and the genitals or anus.
If oral sex (performed by a husband and wife) entails sexual fluids, discharge and impurities to be released, entered into the mouth and or swallowed (which is more than likely to occur), then it is prohibited and impermissible. This is the ruling that all mainstream Islamic jurists agree to, even in the case where one is unsure, however there is a likelihood of such fluids to be entered in to the mouth.
Islam is a religion that demands purity and cleanliness in all aspects of life.
If one engages in oral sex in such a way wherein there is no sexual fluids and impurities on the tongue or in the mouth, then it would be considered mukrūh (disliked). (See Fātāwā al Hindīyyah)
And Allāh Knows Best
Another fatwa (Islamic ruling) on islamweb.net provides more background information about the position of Islam on oral sex:
As for what you mentioned, i.e. sucking, licking or oral sex, there is no clear evidence from the Quran or the Sunnah concerning them. The scholars differed on this matter. Some believe that it is forbidden, others believe that it is lawful. However, Allah Says (what means): {Your wives are a place of cultivation [i.e. sowing of seed] for you, so come to your place of cultivation however you wish.} [Quran 2:222]. This means in any manner that suits you both but in the place of the cultivation which is the vagina. It does not mean anywhere and anyhow. There is a trace from the Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) and his companions that backs up this view. One should be cautious not to swallow some dirtiness, as the man might ejaculate in his wife’s mouth. Allah Says (what means): {Truly Allah loves those who turn unto Him in repentance and those who purify themselves.} [Quran 2:222]. This means those who keep themselves above the dirty and the filthy things, which are the cause of forbidding the menstruation period and any part which is not the vagina. This kind of pleasure is far from the natural dispositions that Allah has created us with. Moreover, a tongue that supplicates Allah and recites the Quran should not get in touch with those parts of the body which are a source of some impurity. Indeed what Allah has made lawful for us is enough for any normal person.
Shaykh Yusuf Badat. Picture screengrab from Facebook video Mathabah