He looked at Gokudera and tried to imagine him with an earring. Then looked back at the display. "Aren't earrings for girls? Or...well...I don't know..." he didn't want to insult Gokudera any, he just didn't understand why he'd want one.
It wasn't really Tsuna's place to tell him an earring was weird and delinquent. So he just reached up, tucked Gokudera's hair behind his ear, and smiled. "Well, I think you'll look really good with or without it, Gokudera." Maybe he'd grow out of the desire.