SPOILER
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Woojin made up with his father, and at the same time Gayoung finished her requirements and graduated. She graduated while Woojin couldn't and had to stay for 2 more years (because of enlistment he had prior on their 1st year). He gave her a proper ring this time as a couple ring and sign of his love for her; also a promise of being together with her until they grow old. It is not necessarily a proposal yet though.
When she gave her the key, it meant she was finally ready to give all herself. This is a reference on the graduation video presentation chapter where Woojin says he can't wait and wants to sleep with her. If you read the side story, it was said that they hadn't done it and because of certain situations, he would wait for her whe she's ready. That also means that the night of her graduation and minutes /hours late after she went home at chapter 98, they had s*x.
I think so, unless Woojin had s*x back in his high school then I think not? Cause like he went to the military when he was 19 (20, kr age) and didn't have his college life until he was 21 (22, kr age).
And Gayoung herself says Woojin was attracted to Yoojin around those time and I don't think he is the type of person to have one night stand with any random person?
Also the fact that he was either with Ohhyung, Gayoung or Yoojin on the year he came back to college (21 years old). He dates Gayoung a little after summer vacation ends and they've been together since.
So yeah, my theory is Woojin was still virgin. As for Gayoung, idk. But the fact that she wanted to prepare herself may state she really is.

Can someone tell me what i just read? I dont get the ending lol