Also, MC really should stop thinking about shit that happened in the past. There's no point. Cherish what you have now in the present. Virginity is overrated crap that's put on a pedestal by useless garbage we call "religion" & "society".
This is definitely one of the best stories in recent months to ride the issue of purity and produced a deliciously bittersweet story. The main draw to this story is that it features a relationship that is most likely going to fail. The girl has shown numerous red flags that make it unlikely for their relationship to work, and the guy has extremely high expectations that are difficult to meet. The only reason they had sex was because the girl was willing to put out for an old friend because she felt sympathy for him (and he also may be going place because he got into a good college).
One or both of them will have to drastically change for this to work out, which doesn't happen that often. The reality is that the relationship will fade after the guy goes to college and accepts who the girl actually is. They'll both then move on to other people.
A little dash of realism in an H-story sometimes works fine. Speaking for myself, once upon a time, any enjoyment I might have had for a story like this would have been soured by Yukiko not "being a virgin". Put simply, I'm over that (IMHO) hypocritical double-standard, where it's pictured as O.K. for only males to have experience.
And what better way for a MC like Souji-kun to "become a man" than with the tender care of an experienced "Sexy Onee-san" who, as it turns out, still has feelings for him enough that she'll wait for him to finish college and come home to her?
That seems to be the frequent theme in similar situations in osananajimi stories I've noticed recently. For some reason, the male childhood friend goes bonkers whenever he finds out his female friend has, or may have had, previous lovers. It's like the males thought they put a licky-stampy on the gal from the beginning of their friendship until when he ever decides to confess.