The first time he saw Nagisa naked was on accident, as he passed by while the boy was pouring water on himself as he sat on a stool. Such a common thing should not have bothered him or his body, and yet he stood there, unseen, watching the young man cleaning himself up.
The second time was also on accident, when Rei walked in on his father guiding Nagisa down his sex, and the latter was all wrapped around such a bigger body, with tension etched on his face and small yelps of pain pouring out of his mouth. Rei was able to leave without being seen, running through the fields outside of home and sitting by a tree for the rest of the evening, unable to get the image and heartache away from himself. However those feelings made him realize that the way he saw the small servant was different from the way he saw any other sweet, harmless boy in his life. Nagisa was peculiar, and maybe Rei had not realized how deep his affection for him had rooted itself.

So the third time he saw Nagisa (almost) naked, that was on purpose.
He gazed at toned muscles, the elegant shoulders and a pretty, slim neck while Nagisa had just began getting undressed for an important dinner at the house, and before he could realize Rei was there, he heard the shōji shut and heard the voice he loved so much call, "Nagisa-kun."
His yukata was still on his shoulders, highlighted so deliciously by the soft sunlight of a time approaching dawn.
"Ryuugazaki-san!" Nagisa chirped, surprised, closing his yukata again, and blushing profusely since, as far as he knew, that was the first time Rei had ever seen any portion of his skin under the clothes. And that belonged to Rei's father, no one else.
But Rei's hand stretched forward, still hesitant, however in the moment his hand wrapped around Nagisa's wrist, the feeling of warm and soft skin pulsating fast under his fingers chased all his doubts away.
"Ryuugazaki-san, I'm sor-"
"Do not apologize. It is me breaking the rules now," Rei replied, with a soft smile, and the fingers holding at Nagisa's wrist softened and then brushed lightly against the back of the other's hand in a gentle, soothing caress. Nagisa's breath quickened, but he seemed to understand, and the shadow of a smile appeared on his lips despite the risks that were all going to lie on his shoulders was he to be found being kissed by the lord's son.
Before either of them could say anything else, their lips were touching softly, a light contact that took both of the boys' breaths away before, in a shower of shivers, Rei's hand snuck under Nagisa's yukata, touching his side and thumbing at his hip bone while tasting the warmth and flavor of the serf's mouth, then dropping fast, messy kisses along his jawline.
Nagisa opened up to him docile, mostly silent if not for small, only choking out yelps of pleasure at his lord's greedy mouth wrapping around him. And just in that moment Rei seemed to realize how long he'd desired Nagisa's body and eyes only for him, looking only at him, without knowing that had been the case since their first day together.
Sliding his hand again under Nagisa's yukata, Rei stood up again from his kneeling position, and his long, slender fingers teased long and excruciating at every inch of the skin between Nagisa's thighs, until they became weak and the small, helpless servant found himself grasping at Rei's clothes, whispering and begging for Rei, "inside of me, please... please."
The moment Rei finally slid Nagisa's yukata from the boy's shoulders, he allowed his thumbs to brush along pale and smooth skin, inhaling sweet scent as the skin is freed and his lips are allowed to explore it bit by bit.
"I'll be late for serving dinner..." Nagisa whispered at a certain point, when Rei's lips were traveling down his collarbone in small, tender kisses, and his hand was teasing at the other's sex again just to see his cheeks flush red and almost feel him melt down under his touches.