I’m not saying they had to love the idea, but was it tolerable?
EDIT : u/sunagainstgold's answer on homosexuality and its perception in medieval Scandinavia is certainly interest you on the other part of your question.
We have litterary evidence that ancient Celts (i.e. more or less people we would call Gauls) practiced homosexuality, to the point it was considered one of their positive qualities by Aristotle.
[...] and yet among the barbarians, a female and a slave are upon a level in the community, the reason for which is, that amongst them there are none qualified by nature to govern, therefore their society can be nothing but between slaves of different sexes. For which reason the poets say, it is proper for the Greeks to govern the barbarians, as if a barbarian and a slave were by nature one. (Politics, I,
So that in such a state riches will necessarily be in general esteem, particularly if the men are governed by their wives, which has been the case with many a brave and warlike people except the Celts, and those other nations, if there are any such, who openly practise pederasty (Politics II, 9)
u/cleopatra_philopater wrote an answer that is necessary to understand what Greeks saw in participating to homosexual and pederastic activities : an initiation of the youngest, wild and "desiring" members of the upper society by older, experienced and guiding mentors. For Aristotle, it was the marking of a comparatively well-regulated and "not that Barbaric" society.
Ancient Celts, however, seems to have overdone it in the eyes of the Greeks as Diodorus Siculus
Although their wives are comely, they have very little to do with them, but rage with lust, in outlandish fashion, for the embraces of males. It is their practice to sleep upon the ground on the skins of wild beasts and to tumble with a catamite on each side. And the most astonishing thing of all is that they feel no concern for their proper dignity, but prostitute to others without a qualm the flower of their bodies; nor do they consider this a disgraceful thing to do, but rather when anyone of them is thus approached and refuses the favour offered him, this they consider an act of dishonour.
Actually revealing in penetration? Boys serching actively for partners? Lack of a natural desire for women as a "settled" man should have? Threesomes? That won't do at all : for Diodorus, regardless of their qualities, these are the displays of a deeply disordered people just as human sacrifices are.
This reputation of Gauls is echoed by Strabo as common knowledge.
[...] one of the things that are repeated over and over again, namely, that not only are all Celts fond of strife, but among them it is considered no disgrace for the young men to be prodigal of their youthful charms. (Geography, V, 4)
Evidently, Gauls had a different approach on sexual practices among men than Greeks, even if it was still marked by intercourse between older mentors and wild youth. Unfortunately, there are no Celtic sources on this; obviously the lack of written litterature in ancient Gaul prevents a direct understanding, but the lack of pictural sources that we could interpret are an additional problems : Gauls didn't represent themselves, except relatively uncommon hieratic fashion, and sexual depictions are usually subtle.
Still, we can draw something more than their immediatly descriptive meaning from Greek texts : at the partial exception of Strabo (who might have borrowed elements from Poseidonios as much from other authors), their perception of Celts was importantly drawn from the form of contacts Greek had with them. In the IVth to IInd centuries it was often as warring bands, either raiders or mercenaries, whom practices including sexual were thus displayed and known to Greeks (earlier mentions of Celts, from the VIth to the IVth centuries are silent on this topic).
We'd be thus allowed to preserve a strong association with pedagogic practices comparable to Greek pederasty, as Aristotle understood it.
We know that warfare was central in Gaulish public life : the social class Caesar describes as equites are determinated by their capacity to wage war as part of the mobilised troop and thus taking an active part into religious rites, political activity and social status among their peers on which personal prestige played the main part until the Ist century BCE. The importance of clientelization and personal relations in the make-up of the pagus (the supra-tribal entity from which troops were gathered and mobilised) would have impressed the pedagogic initiation including warring but also social mentorship and sponsorship that would have included gifts, equipment and sexual intercourse; making fully realized men out of "boys" able to participate to public life as peers; something that could be part of a larger set of practices in Indo-European peoples (altough certainly not limited to them).
In Gaul, it appears from sources that young children weren't fully considered part of the overall social sphere : Caesar inform us that it is innappropriate for a Gaul to bring his child with him, and young children aren't properly buried but cast away in some ditch except in the context of important lineage where they were groomed into a future social role. As such, a pubescent male is immediatly taken from childhood to active initiation, instead of it being a "gateway" of sorts as in Greece;which might have added to the confusion and reprobation from Greeks, but also something that might have well blurred the distinction between "sexually-ehanced" initiation and sexual intercourse among peers, both probably participating from the same set of practices, eventually removing these from strict pederasty and associating virile and social binding among warriors.
The active part taken by the youngers is interesting as it is uncommon and a shocking behavious for Greeks : it might be interpretable as understanding warfare itself as both central for Gauls but as a "special" sphere too. War and mobilisation were made under the auspices of standards taken from sanctuaries, among them the boar-standard or the beastly carnyxes, which had a military and religious meaning : the boar in particular is particularily present in this context while rare in coinage or other displays (where horse are far more common along with cervids).Likewise, Cernunnos as the torque-bearing god and association to cervids could be less a master of nature (and even less a "god of nature") than master of the wild whom initiated warriors (the representation of Cernunnos on the Gundestrup Cauldron being associated with a possible symbolical representation of initiation) could drawn from altough being "harnassed" by the god trough torque-giving.
There's an impression that warfare, decidely distinct from the "normal" sphere (which, incidentally, is governed by women in time of wars, at least for what matter the political management, as accounted by Plutarch), where the wilderness of young men could find an outlet in aggressive warfare and sexuality along older patrons, or as part of a "troop of young men" (or männerbund).
Would these practices have been maintained outside warfare? That's possible but sources are lacking and interpretation of what is at disposal is limited. Warfare being the natural and recurring lifestyle of warriors, we can speculate that there were more than enough occasions to practice something that might have been situated between accepted bisexuality and ritualized pederasty.
Were these practices accepted outside warriors? Opposite stances had been supported and argued, from these being limited to a ritual and "special" context, to Gaulish culture being essentially accepting of bisexuality. Absence of sources (either positively or negatively pointing this) prevent any form of certainty in spite of enthusiastic support for the latter, such as Celts being considered as "sin-free" and "taboo-free" (e.g. Pierre Godard)
[EDIT : We have nothing, on the other hand, on feminine homosexual intercourse or relationships. It was pretty much a non-topic for ancient authors, who focused on a warring elite they were more likely to met with in Gaul or elsewhere in the Mediterranean basin.Altough Gaulish women in particular, and Celtic women in general, seems to have beneficed from a higher status and more agency in public life than in Greece or Rome, we're badly informed on their everyday or public life itself and it is often in relation with how it differed or completed masculine spheres. Even comparatism with other mythological or historical sources (notably medieval Irish and Welsh ones) doesn't give much which could be the sign lesbianism wasn't acknowledged in ancient Gaul either]
What we can say, however, is that Gauls were apparently less uptight than Greeks in their sexual approach (some Gaulish names or surnames have possible explicit translations, such as Tarcondimotus, litterally "his head is like a d**g"), and seems to have, in a context of social upbringing and perpetuation of a male elite, not only tolerated but actively practiced it.