"A literal one" Because his butt wouldn't just be glowing, it would be on fire. It's okay, Gokudera can laugh, Yamamoto is. It's kind of hard not to, it's not every day a person lays on a beach on an alien ship talking about flaming asses after all. It is worth even the teasing to himself to not only let the amusement bubble up in himself but in his friend. He'd argue the fact of being a nice person because he thinks they are all nice people. Even the Cloud in his own way.

Oh hey, if he does not open his mouth in the next three seconds he may slide through this. Nope, opens mouth, inserts foot "They'd be hard to resist"

Meaning at some point, he would try. It's Yamamoto, personal space is not always something he thinks of. He has taken a lump or two to invade it without cute little cat ears after all. Yes, he is very cat-like and that only adds to the ears, it's amusing enough that despite the unimpressed look on Gokudera's face this baseball idiot is still grinning.