They didnt just fight unarmored, they foguht butt naked, they believed it gave them the blessing of their gods, who would view them as courages. Which is not to say that Mainland celts lacked armor equipment, compared to their germanic neighbours, they didnt lack Iron (Noricum was pretty much where rome would get all its iron from later on) or the techniques to forge proper armor (Roman helmets for example were a Gaulish design)
They chose to do so And it terrified the Romans
And here how fighting naked turned out
Very terrifying too were the appearance and the gestures of the naked warriors in front, 8 all in the prime of life, and finely built men, and all in the leading companies richly adorned with gold torques and armlets. 9 The sight of them indeed dismayed the Romans, but at the same time the prospect of winning such spoils made them twice as keen for the fight. 30 1 But when the javelineers advanced, as is their usage, from the ranks of the Roman legions and began to hurl their javelins in well-aimed volleys, the Celts in the rear ranks indeed were well protected by their trousers and cloaks, 2 but it fell out far otherwise than they had expected with the naked men in front, and they found themselves in a very difficult and helpless predicament. 3 For the Gaulish shield does not cover the whole body; so that their nakedness was a disadvantage, and the bigger they were5 the better chance had the missiles of going home. 4 At length, unable to drive off the javelineers owing to the distance and the hail of javelins, and reduced to the utmost distress and perplexity, some of them, in their impotent rage, rushed wildly on the enemy and sacrificed their lives, while others, retreating step by step on the ranks of their comrades, threw them into disorder by their display of faint-heartedness. 5 Thus was the spirit of the Gaesatae broken down by the javelineers;
TL;DR, they were easily killed.
But Rome fighting for patriarchy? Thats utter nonsense.