Well, Riku didn't think it was gross if that was what you were into... but it was gross to think of himself in that kind of relationship. I mean, a boy? Short hair, flat chest, no curves, hairy bodies, sweaty, smelly boy? Not to mention that said boy would want to touch him, which wigged him out, and Riku had no interest in a boy's body anyway.
"Too much of a boy?" Riku repeated, and grabbed Sora around the waist, starting to tickle him. "When boys date boys they usually WANT them to be boys!" And, you know, not girls. And he couldn't even imagine being in a relationship where someone tried to treat HIM like the girl. He would NOT stand for that.
If, Gokudera figured, his brother was SO uninterested in boys, why was he putting his hands all over Sora right now? You didn't NEED to tickle someone to object to their logic.