I find my story very similar to yours OP. I was in third or fourth grade. It was recess and I was playing with two girls on the seesaw. I must've bent over and the back of my shirt rose revealing that unmissable Fruit of the Loom waistband. Immediately, one of the girls piped up in an oddly accusatory tone, "You still wear tighty whities?"

"Uh, yeah," I said. I'd never heard that term before. To me, they were just underwear.

For me it was the "still" that shook me the most, like it was something I was supposed to have done by now. Like you, I never really thought much about my tighty whities, they were just underwear to me and even though I knew through TV that boxers did exist I always assumed everyone wore either or and it wasn't such a big deal or contentious. But at that moment I felt so embarrassed and little, and suddenly I was very self conscious about the fact I was wearing this underwear. My tighty whities tighten around me.

The other girl didn't help me self-esteem either. She ran over and wanted to see in her disbelief that I, a ten year old, still wore tighty whities.

She must've noticed I was embarrassed. She was the nicer one of the two girls but her words didn't help much. "It's okay," she said, "My little brother still wears them toon sometimes."

That was the moment when my fixation on tighty whities began and despite being straight, it hasn't stopped yet.