I haven't found any scriptures which explicitly forbid this. So my questions are:
- Is oral sex involving male genital organ that is totally consensual with the said person's wife a sin?
- Is oral sex involving female genital organ a sin?
Oral sex, also known as "Auparishtaka" is described in many Hindu scriptures.
The Srimad Bhagavatam 5.26.26 strongly condemns oral sex:
यस्त्विह वै सवर्णां भार्यां द्विजो रेत: पाययति काममोहितस्तं पापकृतममुत्र रेत:कुल्यायां पातयित्वा रेत: सम्पाययन्ति ॥ २६ ॥
yas tv iha vai savarṇāṁ bhāryāṁ dvijo retaḥ pāyayati kāma-mohitas taṁ pāpa-kṛtam amutra retaḥ-kulyāyāṁ pātayitvā retaḥ sampāyayanti.
If a foolish member of the twice-born classes forces his wife to drink his semen out of a lusty desire to keep her under control, he is put after death into the hell known as Lālābhakṣa. There he is thrown into a flowing river of semen, which he is forced to drink.
The Vashistha Dharmasutra 20.20 prescribes an expiation for Brahmanas who swallow semen:
If (a Brāhmaṇa) intentionally (drinks) other spirituous liquor than that distilled from rice, or if he unintentionally (drinks) spirituous liquor extracted from rice (surā), he (must perform) a Kṛcchra and an Atikṛcchra, and, after eating clarified butter, be initiated again. The same (expiation is prescribed) for swallowing ordure, urine, and semen.
Part II, Chapter 9 of Vatsyayana's Kamasutra describes oral sex as a sexual activity performed by eunuchs and unchaste women.
The Auparishtaka is practised (other than eunuchs) also by unchaste and wanton women, female attendants and serving maids, i.e. those who are not married to anybody, but who live by shampooing.
The Acharyas (i.e. ancient and venerable authors) are of opinion that this Auparishtaka is the work of a dog and not of a man, because it is a low practice, and opposed to the orders of the Holy Writ, and because the man himself suffers by bringing his lingam into contact with the mouths of eunuchs and women. But Vatsyayana says that the orders of the Holy Writ do not affect those who resort to courtesans, and the law prohibits the practice of the Auparishtaka with married women only. As regards the injury to the male, that can be easily remedied.
The Baudhayana Dharmasutra forbids all forms of non-vaginal sexual intercourse (which includes oral sex):
A man who ejaculates his semen in any place other than the vagina becomes equal to a thief and to a murderer of a Brahmin. (Baudhayana Dharmasutra, Praśna III, Adhyāya 7)
The Gautama Smriti 25.7 makes a similar statement:
He who has been guilty of cheating, of calumniating, of acting contrary to the rule of conduct, of eating or drinking things forbidden, of ejaculating inside a Shudra woman or in any place other than the vagina, and even of performing magic rites with intent (to harm his enemies), shall bathe and sprinkle himself with water, reciting the texts addressed to the Waters, or those addressed to Varuna, or other purificatory texts.