Within the guidelines of halacha, as presented by chazal and early rabbinic authorities, fellatio is classified as ביאה דרך איברים (penetration by way of the limbs) or מעשה חידודים בעלמא (general acts of sharpening).
Peripheral halachic discussion of the topic revolves upon whether a husband may engage in varying sexually-gratifying acts with his wife – including fellatio – performed up to ejaculation.
The presentation omits the customary prohibition on extracting semen in vain, which HaRav Yossef Qafih quarantines as an addition by a subsequent unknown editor.
A more explicit permissive stance is that of the tosafist rabbi Isaiah di Trani the Elder who hints that a complete extra-vaginal ejaculation is permitted whenever needed to "seat" one's desire with the stark exclusion being to avoid pregnancy; What was the (forbidden) action of Er and Onan that the torah prohibits?
[9] Later rabbinic authorities differentiated between masturbation (self-extracting semen in vain) and an extra-vaginal ejaculation achieved with the active participation of one's Torah-permitted partner—with the former being forbidden under most if not all circumstances.