I think when people say that sex in yuri manga is the "end goal," they're not referring to the act of the characters having sex as the ultimate end goal. They're referring to when the AUDIENCE gets to SEE it. Going back to Kase-san, it's no coincidence that they waited until the final chapter of the manga's -INITIAL RUN- for them to do it. (Remembering that "Yamada and Kase-san" is a sequel, not the initial series) Multiple ships in Strawberry Panic are seen having sex only towards the tail end of the series, even if they have explicitly been mentioned if not shown to have had past sexual history. In Girl Friends, Akko and Mari don't have sex until the very end of the series. And the list goes on.
While it's true that in Japan and many other parts of the world, sexuality is considered "impure," and virgins are cherished for their perceived "innocence," I believe that's only one part of the problem here. Sex isn't withheld solely because of its connotations as a perceived loss of innocence, but also as a metaphor. In virtually every romance, the end goal is for each member of the relationship to come to a mutual understanding. When after many long chapters of conflict, the two characters finally come to an understanding and express their mutual love through intimacy. During intercourse, their bodies and their souls are laid bare, so that their partner may truly understand them, literally inside and out. The members of the relationship come to understand how the other feels, what they're really hiding beneath the mask and layers of clothing and/or makeup.
It's an effective metaphor, I think, and a powerful statement of how simultaneously simple and complex a relationship can be, having layers that are both primal and relatable to virtually every human while also having unique, introspective layers that only the participants can truly understand.
My problem isn't really with the idea of sex as an end goal for the reader (though it is a delicate balance that can easily be done wrong, and has been done wrong on countless occasions) as much as the unrealistic portrayals of sex that seem to be more catered toward making it visually appealing than actually integral to the plot. The mass media in general (in ALL parts of the world) tends to show sex as just the best gosh darn thing ever and if you haven't had it yet, then wow you're really missing out! And if you ever had a bad sexual experience, then you're going to be the laughing stock of the entire Girl Community forever!! What I'd like is if more shows demonstrated a first time experience with sex as how it normally is: That is, awkward, often uncomfortable, but nevertheless a learning experience. Not something that fundamentally changes who you are or your place in the world, but just one of the many stepping stones you will encounter as you grow and develop as a person.
And in my opinion, more than just serving the narrative, leaving in the nuances and awkwardities of sex makes it more appealing even if all you're looking for is something to get off to. It adds dynamic and charm to the encounter, gives you more thoughts and ideas to explore, etc. Because humans aren't perfect and sex is done by humans, then sex shouldn't be, either. It should have ups, downs. Funny moments, heartwarming moments, sometimes it can be sad, sometimes it can be contemplative. There's just so much you can do with sex as a plot device aside from "oh yeh feels so gewd." It feels like a sort of waste of narrative potential, especially in a series like Kase-san, where exploring one's sexual identity is such a prominent aspect of the characters. And Kase-san is one of my favorite manga ever, so I don't say that to put it down, just to note that it could have been better in this regard, as could many of my favorite yuri stories.
