The great jurist and muhaddith of this era, Hazrat Allamah Mufti Ahmed Khanpuri (May Allah preserve him) writes in his fatawa:
“It is not permissible for a woman to take the private part of a man in her mouth and lick or suck it. It is also not permissible for the husband to instruct her to do so. The mouth is a noble body part. In it is the tongue which is used to recite the zikr of Allah. How can it be permissible to use it in such a disgusting act? Especially since the hadith has explicitly prohibited us from using our right hand to touch our private part at the time of istinjaa.” (Mahmoodul Fatawa 5/108)
The following will elucidate this extremely strong evidence concerning the issue under discussion:
In the same manner, that Allah Ta’ala has endowed the human with nobility, He has also given certain body parts virtue over others. In Sunan Abi Dawood (32) it is narrated that Hafsa (رضي الله عنها) said: “The Messenger of Allah (صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ) used to use his right hand for his food, drink and clothing, while he used his left hand for other than that.” In another narration, it states: “His left hand was for the toilet and anything that was filthy.” (Abu Dawud #33) Allamah Mahmood Subki Maliki mentions in his Sharh on Sunan Abu Dawud, that touching the private part with the right hand is not only disliked while cleaning oneself, rather it is general for all occasions based on the generality of this hadith. (المنهل العذب المورود 1/120-121)
.عن حفصة زوج النبي صلى الله عليه وسلم أن النبي صلى الله عليه وسلم كان يجعل يمينه لطعامه وشرابه وثيابه ويجعل شماله لما سوى ذلك
.عن عائشة قالت كانت يد رسول الله صلى الله عليه وسلم اليمنى لطهوره وطعامه وكانت يده اليسرى لخلائه وما كان من أذى
The Messenger (صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ) also explicitly prohibited his ummah from touching their private parts with their right hands. Abu Qatadah (رضي الله عنه) said: The Prophet (صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ) said, “Do not touch your private parts with your right hand while urinating, nor should you use your right hand for washing or cleaning (your private parts).” [Bukhari #168 Muslim #268]
“عن ابي قتادة عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، قَالَ : ” إِذَا بَالَ أَحَدُكُمْ فَلَا يَأْخُذَنَّ ذَكَرَهُ بِيَمِينِهِ ، وَلَا يَسْتَنْجِي بِيَمِينِهِ ، وَلَا يَتَنَفَّسْ فِي الْإِنَاءِ “
Since it is proven that the limb which is used to place food in ones’ mouth, the right hand, has been endowed with honour from Allah, and it is not permissible to touch one’s private part with it, then how can it ever be permissible to desecrate and degrade the actual mouth which is used to proclaim the kalimah taiyibah (لا اله الا الله), to make zikr of Allah, to recite the holy Quran and to eat through oral sex?
Hazrat Mufti Saheb had also mentioned that one uses his mouth to recite zikr. The recitation of the Quran Majeed is a form of zikr. Ibn Majah narrates that Ali [رضي الله عنه] said: “Your mouths are the avenues (for the recitation) of the Quran, so keep them clean with the siwak.” (# 291) This has also been narrated marfoo’an from the Messenger صلى الله عليه و سلم. (See التلخيص الحبير 1/116) When Abdullah bin Busr [رضي الله عنه] asked the Messenger of Allah to inform him of an action that he should stick to.” He said: “Let not your tongue cease to be moist with the remembrance of Allah.” (Tirmithi #3375, Ibn Majah # 3793) Who can deny how clean the Messenger of Allah used to keep his mouth. The ahadith concerning his usage of the siwak are mutawatir. Therefore one should keep his/her mouth perfumed and clean at all times.
In conclusion, saying we are not allowed to touch our private parts with our right hand but we are allowed to touch it with our mouth is truly nonsensical.