Original Korean Webcomic https://www.lezhin.com/ko/comic/she_is_young Translated by Game of Kekeke http://kek.gameofscanlation.moe/
Duksin Park is tired of the sex with his girlfriend of 10 years. He feels that there is no excitement as she doesn't have any shame. Then he suddenly sees a girl who claims to be his girlfriend and looks exactly likes her when she is in 3rd year of high school.
Man.. I wish we had more H series/endings like this. Right in the feels. :') Series with sex and nothing else are great, but I wish more, esp ones with a story, went like this.. ending and all. (i.e no rape, forced (i.e. 'betrothed to' or "if you really want that promotion" type stuff), mindbreak, corruption, blackmail, NTR, cheating or the classic porn/hentai trope, everyone has to be a slut/man slut, anyone who isn't at the start, must be by the end. (even if through one of those ^) As well as the drama that goes along with that crap a lot of the time. (with long storied series at least)
And OMFG at that threesome! :D He is one LUCKY SOB, and that's even with only one of them, but BOTH!! *drools*