The act of kneeling to a superior is a common display of reverence even in the ancient world. For the defeated Gallic leaders this was intended as an act of humiliation to settle Roman superiority, but how conscious were the participants of the sexual connotations? Were Gaul clansman humiliated because it symbolised utter defeat, or because it was seen as representing oral sex and sexual submission?
I guess I want to better understand whether sexual psychology was a conscious part of the Roman war on Gaul, or whether it only feels sexual now because of the popularization of Freudian psychology.
What is your source for this? I've checked all the major sources and came up totally empty. The most famous surrender of a Gaul to the Roman's, Vercingetorix, is undoubtedly what you're thinking of. He's considered archetypal in the popular imagination for reasons that are beyond me--he is not presented as such in the sources at all, but rather quite unusual. Plutarch, Suetonius, and Caesar nowhere report such a practice. Plutarch says that when Vercingetorix surrendered he put on his best armor, rode alone to the Roman camp, removed his armor and dropped his weapons, and then prostrated himself before Caesar. Suetonius says nothing. Caesar simply says that he awaited the surrender of the defenders at Alesia from a seat in front of the camp, and that the Gaula deposited their weapons. Indeed, this is usually the formula for surrender in the de Bello Gallico. Caesar will call out the enemy to surrender, and they will deposit their arms. Often Caesar will also take noble hostages. It's an important point, because in the first major siege of the Gallic war, against the Aduatuci, the Aduatuci pretend to deposit their arms but actually hide a lot of them in a pit in their fort.
It's possible that there's a source that I haven't looked at, but I'm skeptical. What's your source on this? If this is attested it cannot be normal practice, nor is it likely to have occurred with Vercingetorix. But I feel pretty confident in saying that it's not attested, although I'd it is I'll cross that bridge when I come to it. It would be a bizarre practice--the veneration of the eagles as a symbol of the state is attested among the soldiers in Tacitus, but not as a general social thing and certainly not to my knowledge as a way of imposing sovereignty over conquered peoples
There's no account of a systematized and ritualized submission of Gaulish magistrates, kings or nobles to be found in ancient sources on Gallic Wars and critically the Commentaries on the Gallic Wars : Vercingetorix's surrender, after what was unmistakably the most important battle of the Caesarian campaigns, is barely described at all^(1) (Plutarch arguably gives a more grandiose description, but definitely favoring Vercingetorix's nobility.) in the same time Caesar wanted to gain, or at least aggregate to Romans, various Gaulish peoples it generally passed trough some clemency and hostage-taking (especially for people with a traditionally strong pro-Roman faction), and an harsh treatment from forcible hostage taking to enslavement for the others.
The political and strategical goals of Caesar, indeed, wasn't to humiliate defeated leaders and enjoying some kick out of it, but to conquer a region defined by its multiple polities and an important population. Humiliating potential (and former) allies or clients would be an hindrance pacifying the region : being sensible and avoiding the pitfall of tyrannical governance that defined Transalpine Gaul in the late Republican era was what allowed Gaul to be fairly peaceful during the Civil Wars and the transition between Caesarian and Augustean rule, in addition allowing Caesar to display an important Roman virtue, moderatio, which was what separated a Roman from a Barbarian.
It doesn't mean that there wasn't humiliating punishment of Gaulish nobles, especially the execution of Acco tried and punished by Caesar, in a Gaulish assembly (although possibly an assembly gathered and conceptualized by Caesar rather than the traditionally gathered assemblies in Gaul) but in roman fashion and in at least formal accordance of allied peoples; Acco being guilty for Caesar to have organized a rebellion in central Gaul. Similarily, the description of Caesar's wrath against Vercingetorix when he went to meet him (again in a reasonably noble fashion), opposed to the advice of his allies (probably Gaulish ones) to show a particular clemency and as for Acco in -53 effectively accusing and judging the former commander-in-chief of the Gaulish coalition as a rebel against Gaul and its Roman protectorate.
What was at stake was a Roman trial and condemnation, but accepted and with the active participation of Gaulish elites, just as with Acco, and as it happened with the gutuater of Carnutes in -50.
Furthermore, it would be a strange use of the roman aquila : rather than symbolizing the roman state or rule, it was an army standard having a dual sacred and symbolic function distinct from political leadership, as illustrated by the mutiny of Germanicus' soldiers ready to kill a senatorial envoy that was spared this fate by holding on the eagle and standards.
As far as I can tell, this description would come from "The Stolen Eagle", the first episode of the HBO series Rome. I couldn't find evidence for the historical reality or assumption of this particular depiction, and it's likely it could rather be attributed to what became the "gritty" trademark of historical or "realistic" fantasy such as Rome, Games of Thrones, Spartacus : Blood and Sand; etc. maybe telling us more about our own media culture than on Gallic Wars (which we could charitably qualify, giving Gauls are not the focus of the show, as a disappointing but expected poor reconstitution)
^(1) The next day, Vercingetorix gathered his council and said he didn't undertake the war for his own sake but for the cause of common freedom, and since he must yield to fortune, he offered his person to them either to satisfy Romans by his death or by surrendering him alive. Delegates were sent to Caesar about these things. He ordered that they surrender their weapons and deliver their leaders. Setting himself before the fortified lines of his camp, the commanders are delivered; Vercingetorix surrendered, weapons thrown away. At the exception of Aeduns and Arverns, that he could regain their cities trough them, the other captives were distributed to each soldier of the whole army, as plunder.
^(2) Caesar precises (DBG, VI,44) he was executed following "the custom of our ancestors", which probably means being bound to a post, whipped and beheaded.