I don't think Rui can find anything Tsukasa does extremely underwhelming tbh, I think he would be extremely turned on by Tsukasa being quiet during sex. It's way too moe, how shy Tsukasa gets during intimacy. He blushes so much and tries to contain his moans covering his mouth with his hand, but Tsukasa only blushes even more when Rui takes his star's hand out of his mouth and his him a kiss on his palms… It's too cute how Tsukasa is quiet during sex, because it's something only Rui gets to see and hear. He loves hearing Tsukasa short and broken breaths, the way Tsukasa whispers Rui's name for him to be closer, for their skins to touch... too cute, he is overwhelmed with happiness with this new discovery.