The Celts were contemporaries of ancient Greece and Troy, and they lived in Anatolia in a neighborhood called Galatia (what is now Turkey), plying a prosperous trade with both city states until Helen mucked up everything with her affair with Paris.

This scene depicts Greece's greatest warrior, Achilles and his best friend, Patroclus who was killed by Hector the Trojan. Achilles subsequently killed Hector (and dozens of other folks) in revenge. There is some suggestion that Achilles and Patroclus were comrades in arms in more that one sense: (see excerpt below), but Achilles also killed a lot of people because he was angry at Agamemnon for stealing his female prize, one Brises with whom he also had relations. The Celts played a minor role in the Trojan war, but they liked fighting and loving every bit as much as the Greeks and Trojans.
— Excerpt from "The Celts" by Gerhard Herm, pg 57-58

The Celtic women [Diodorus writes] are not only as tall as the men, but as courageous…But despite their charm, the men will have nothing to do with them. They long instead for the embrace of one of their own sex lying on animal skins and tumbling around with a lover on either side. It is particularly surprising that they attach no value to either dignity or decency, offering their bodies to each other without further ado. This was not regarded as at all harmful; on the contrary, if they were rejected in their approaches, they felt insulted. Strabo confirms these homosexual practices with the brief mention that the young men of Gaul are shamelessly generous with their boyish charms; while Athenaeus, another of the writers who dug deep in the treasure trove of Posidonius, also says that the Celts generally slept with two companions in a bed - and not because the winter was very cold.

We can therefore suppose that this was a warrior society strongly characterized by man-to-man bonds. As soon as they were old enough to bear arms, young people lived almost wholly with others of their own sex. They learnt riding, swordsmanship, hunting, drinking; they had to prove themselves in the field, were (or were not) honored at the feast, and saw their like as the only suitable company. It is not hard to see that this placed enough emphasis on the latent homo-erotic component of many male friendships to turn them into true homosexuality. His comrade became the object of admiration and desire for a man, his driver for the passenger, his spear carrier for the warrior. Thus Achilles loved Patroclus as did Alexander the Great Hephaestion. Wherever there was no taboo, such relationships understandably gave rise to a cult of the male body.