He didn't actually do it for any reason other than the fact that second-hand shops had a lot of female clothing. Sometimes, it was hard finding cheap clothes that fit, so Prompto had stopped caring about what sex they were made for a while ago. As long as it fit, and it was cheap enough, he would buy it.

Prompto had been doing this all his life, especially when he was losing weight and had to buy a new wardrobe for himself. That's when found out how fucking expensive clothing could be. Who knew a T-shirt and jeans could cost upward of 200 gil? He got over any lingering shame about gender norms a while ago in the face of cold hard reality. Prompto really couldn't afford to blow all his money when he had bills to pay, food to buy, and new camera lenses to purchase.

As they say, 'the people who care don't matter, and the people who matter don't care' or however the saying goes. And Noctis sure never seemed to give a damn whether he was wearing a t-shirt that said "Bitch, Please" or one that had a pinata with the caption "I'd hit that".
So he doesn't think anything of showing up at Noct's place in a tube top.

"Prompto?"

"Yeah, dude?"

"What are you wearing?"

Prompto glanced over at his friend in confusion. Noctis was no longer staring at the game in front of them. Instead, he was staring at Prompto with wide eyes.

"What do you mean?"

"Your, uh, your shirt."

At that, Prompto glanced down at the aforementioned shirt. It was a plain black tube top that went down over his cargo shorts. Prompto didn't see anything wrong with it.

"What about it?"

Noctis was at a loss for words. He opened and closed his mouth a few times, cheeks dusted pink, unsure how to word his flailing thoughts.

"Why… isn't that… for girls, or something? Why do you have it?"

Prompto stared at his friend in silence and Noctis felt his face heating. Had he offended the usually bubbly blond? Noctis' face went bright red when Prompto burst out laughing.

"What's with the face, Noctis? Are you embarrassed or something? It's not like I got any boobs to show off," he giggled.

"Sh-shut up! I'm not embarrassed it's just—why?" Noctis stammared.

Prompto let is remaining giggles fade, though his grin was firmly stuck on his face, much to the prince's irritation.

"Well Noct, I don't know if you realize, or if you even go clothes shopping, but clothing is really freaking expensive, and I really don't have that kind of money. So, I usually go to the thrift store and buy whatever happens to fit. If that happens to be tube tops and skirts, well, at least it's cheap."

Noctis didn't know what to say to that. Sure, he knew his friend carefully monitored how much he spent whenever they went anywhere together, but he hadn't realised the budgeting extended past frivolities.

"So," Prompto's voice cut into his thoughts. The blond had a shit-eating grin on his face. "Do you want to see me in a skirt?"