Sheikh `Abd al-Wahhâb al-Turayrî says that the Prophet is not against oral sex as he makes no mention of it. Anything that is not clearly opposed or made unlawful by the Prophet is permissible.
The Shafi`is and Hanbali doctrines consider sperm to be pure but state that it is unlawful to swallow the fluid.
If the bodily fluids aren't swallowed then oral sex is no different from kissing various parts of the body, which is allowed for Muslims. But couples may not indulge in it to such a degree that oral replaces normal intercourse.
The Hanbali followers of Islam say that even though Muslim couples don't swallow seminal fluid, those who participate in oral sex should always rinse their mouths after the act as part of Islamic purification.