Well it's the same Shuun from Hadakeru Kaibutsu. Isn't it? They are both named the same, they look the same and they're both huge down there. So this is why, it's hard for me to totally appreciate this manga since I feel like its the same Shuun that's having a true romance with another man years later after college.
Its not the same guy, also Shunna ( the one from hadakeru) , clearly states on this page http://www.mangago.zone/read-manga/hadakeru_kaibutsu/mi/e4de88c2dd055b9b45ff0d9d4efa4e61_chapter-1/18/ that his first time with a man was with Kan-san, on the pages before this one he also says he has always been with girls ONLY. Shunna is heterosexual, he likes girls, he never loved any of them though, but love is one thing lust is another, now we could say he is bisexual( after meeting Kan-an). While shun-chan says he is gay, girls dont do for him, he only dated girls as cover as you can see here http://www.mangago.zone/read-manga/sayonara_futaribocchi_renai_shinsho/nbt/1f761ed998c2ed69/Ch1/11/
Shun-chan is way taller, he is freaking 1,90cm !! Shunna is like 1,82 or something. Shunn-chan's ear is pierced, Shunna's isnt. TBH if you reread Hadakeru's first chapter youll remember other things that Shunna says that makes it impossible for him to be Shunn-chan. And yeah as stated above their names are different. About their faces being similar yeah, they are thats a common thing in most mangaka's works, their characters from different stories look similar, if you ever read Natsume's works youll notice it too. Its like they get used to draw only two types of faces or something lol I totally get you.
The only reason I can't fully enjoy this manga is because Shuun ended up with Yumi's senpai in the other manga. It bothers me. A LOT. What about Yuuta? What happened?