it's time 
It Starts From the Inside (Michinoku Atami)
Sho finds out what his boss, Noriyuki, is into and he takes sweet, sweet advantage of that.
yes this is a manga about sounding and yes i came for the smut but i stayed for the romance, because they get genuinely so sweet as their relationship progresses 
one of my favorite ones to reread 
Sho Itoguchi, an office worker who runs a site selling urethra bougies as a hobby, finds his boss's name, Noriyuki Kiyama, within the list of prospective buyers. Later on, he brings some urethra bougies to Noriyuki's home and starts his "lecture" just like that. Despite it being Noriyuki's first time, he's drowning in pleasure in front of Sho. Sho can't hold back his excitement...!
Amara