KTL has summed it up excellently MashaAllah.
Islam defines Aadaab with respect to every limb. For instance, eating with the left hand is against the Sunnah because it is used to wash the private parts, used to clean the nostrils during wudhu, etc. If the left hand is against Adab in order to eat with then imagine the gap between oral sex and Adab of the mouth and tongue which is used to Praise Allah Subhaanahu wa Ta'ala and is used to perform clean and pure acts. This is logic for the intellect which will not understand the rulings as such.
Every act has its adab. From what I can definitely assure you is that this is not an act practised by the pious and Awliya Allah.
A Shaykh once referred to themself and told their disciple that they had not been near their wife (referring to the specific obligation of a husband and wife to each other that becomes permissible through Nikah). This dumbfounded the disciple because the Shaykh had children. Later they explained that whenever they went to their wife, it was with the pure intention to perform the Sunnah (and fulfill her right and their duty) and not for bodily desires, hence, they didnot go to their wife for their nafs and this is what they meant when they said that they had never went near their wife.
If we keep these standards as our ideals, they will guide us and teach us Aadaab pertaining actions. Permissibility is not the only factor considered when seeking guidance pertaining acts which hold no reward i.e. mubah, makrooh tanzihi and tahreemi, etc.
A Muslim saves himself from the haraam and keeps himself away from it as much as possible and not let himself on the thinline between 'permissible yet non-rewardable' acts and sinful actions.
In order to secure one's actions of Faraa'idh and Waajibaat, he has to be steadfast on the Sunnah. In order to secure the Sunnah, one has to be steadfast upon the mustahibbaat - rewardable acts. The latter protects the former. A person who tends to not care about his makrooh and mubah acts will tend to fall in the trap of shaytaan easily. Such person's good deeds are not protected. This is what is required to be considered when seeking rulings and not an excuse to go ahead with such acts.
Makrooh and mubah donot mean a person is safe to practice these acts. This is a thinline between reward and sin. These are acts of carelessness and this quality is of the weak ones. The rulings are to define the thinline so that the Muslims keep themselves away from these acts because the next lower step is committing a sin.
Hence, the complications in oral sex are very vivid. The reason why it is declared makruh i.e. unrewardable is because it is a thinline between sin and an unrewardable act. Sin because swallowing semen is haraam and oral sex is makrooh. No benefit either way. There is no guarantee that a drop of semen will not enter the throat. Every sane person can reflect it with the passion during such moments. However, it is rewardable to fondle the private parts of one's spouse with the intention of pleasing each other.