Fabric store joke seemed a bit forced. There's no reason the guy should have assumed the dude was making the costume for himself, especially if he was set in his ways about gender roles. I'm assuming in Japanese it was one of those cases where what was said can be easily misinterpeted?
It's honestly refreshing to see a teenage MC with a healthy libido that isn't a pervert. It feels like in most cases it's either 'huge pervert', 'gets embarrassed but not much else', or 'I couldn't care less about your naked body'.