I think it's like a dam bursting open. Because Shirotani never really touch anyone before, nor was he ever touched, and he keep suppressing all his desires. So now that he let it out, there's no way he could control it. merely being close to his boyfriend or thinking of him will make him hard. he's like a freakin teenager all over again XD
If this a yaoi manga, Miyamura would be the total cutie pie uke... and Hori is the sadist seme LOL. well, I love them as girlxboy too of course. Miyamura might not be the tall, dark, and perfect bf, but he's a dreamy one...
oh, and I love him with his hair tied up in the earlier chapters. He looks cool that way.
So I finally read this long after reading Merry Checker LOL. it's good fun too. For those who don't know, there's a spin-off where Mitsuya made cameo appearances in a few chapters. It's called Merry Checker where the main char is the senpai who he asked about 'love'.