Originally posted 2019-10-01 11:14:52.
In the name of Allah, We praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright.
Almighty Allah says:
Your wives are a place of sowing of seed for you, so come to your place of cultivation however you wish and put forth [righteousness] for yourselves. (Al-Baqarah 2:223)
In the Hadith, the Prophet (peace be upon him) said, “In man’s sexual intercourse (with his wife) there is a charity.” They (the Companions) said: “O Messenger of Allah, is there reward for him who satisfies his sexual need among us?” He said, “You see, if he were to satisfy it with something forbidden, would it not be a sin on his part? Similarly, if he were to satisfy it legally, he should be rewarded”. (Muslim) 
It is permissible for a husband and his spouse all forms of sexual pleasure provided there is mutual agreement between the spouses and there is nothing brings harm to either of them; except for anal sex, and during menses and post natal bleeding.
Touching ones spouses private parts or vice versa with mouth is what is known as oral sex. Such an act is obviously unnatural, unhealthy, unhygienic, unclean, impure and this act is not found even in any of the lower animate creations!
But since the permissibly / impressibility of this specific act of what is known today as oral sex is not mentioned by name or act anywhere in the Qur’an and the Sunnah, the Allah-fearing and the pious amongst the scholars in Islam have abstained from declaring it haram (unlawful); the most they have said is that the act is from the khabaith (impure) and thus closer to the haram than to the halal (permissible), and the believers should fear Allah and abstain from it.
As far as Taharah (purification) from male secretions is concerned, ghusl (ritual bath) is obligatory only in two cases: ejaculation of sperm and sexual intercourse. Any other thing that does not end in ejaculation of sperm or having sex does not require ghusl.
If one releases Madhi (pre-seminal fluid or ejection), then wudu’ (ablution) is only obligatory. In case of sexual intercourse, ghusl is obligatory even if one does not ejaculate sperm.
If nothing is discharged, it is better for them to make wudu’ because of touching the private parts as well as to avoid the potentiality of the discharge of madhi or wadi during the oral sex.
As for the wife, she is not required to perform a ghusl unless she has an orgasm and discharges sperm by merely practicing oral sex on her husband, in which case she is required to perform a ghusl, or discharges fluid [without an orgasm], in which case she is required to perform ablution. She must also wash her mouth thoroughly if fluids come into contact with her mouth.
In the case of a husband performing oral sex to his wife (cunnilingus), if the latter reaches sexual climax (has an orgasm) and discharges sperm, a ghusl becomes necessary on her. If she discharges fluid not sperm, then only ablution is required.
As for the husband, he is not required to perform anything unless he ejaculates sperm by merely practicing oral sex on his wife, in which case he is required to perform ghusl, or releases the pre-ejaculation fluid (madhi), in which case he is required to perform ablution.
However, couples should bear in mind that it is impermissible to intentionally swallow sexual fluids or introduce them into the mouth.