Mitsunaga Ougo is Trying to Control Himself summary:

"When we turn 18..." In order to fulfill his cute girlfriend, Karen's wish (?!), Mitsunaga has been holding back a lot. He may be a quiet, hard-headed guy, but he has as much sexual desire as anyone else, and was even looking forward to it a little bit. However, on the anniversary day, something terrible happens?! He can't touch her because she's precious to him, but he wants to touch her more because he loves her... A pure love story about the troubles of a boy of age!
This is the sequel to the volume, "Karen Ichijou Tempts Him"