I appreciate your argument, though I can't really get behind your assessment of the finale as "a plot-driven bore." The larger story-arc is character driven, but there IS an awful lot of plot to wrap up in book 7. Speaking just about my personal experience of the book, rather than its literary merits--which I haven't yet had the time to step back and analyze objectively--I can't say that I was disappointed in that.
As for the lack of sex...well, first off, I have to say, I think Ginny was planning on giving Harry more than a kiss for his birthday, had they not been so rudely interrupted...and considering the continuous mortal peril in which the characters find themselves, a fair amount of "snogging" DOES go on in those closing Hogwarts scenes.
However, you're right: the long months that Harry, Ron, and Hermione spend on the run are notably devoid of sexual tension. Until you mentioned it, I have to admit, it did not strike me as unrealistic. Your observation prompted me to delve a little deeper into that reaction. WHY didn't it strike me as odd?
For one thing, these characters are forced to grow up very quickly, and with the weight of the world on their shoulders. It was easy to forget that they were teenagers.
For another, there was the personality-altering influence of the Horcrux, coupled with the realities of stress, hunger, and frustration. I had no trouble accepting that these things would dominate the characters' experience and color their interactions.
I think sex also took a back burner for me because for a large percentage of the journey, Ron was absent, and he was the only one with a romantic interest in Hermione. Harry is still carrying a torch for Ginny (not to mention a Horcrux and a whole lot of stress), and has always treated Hermione more like a sister, so I would have found it unrealistic if sexual tension HAD emerged there.
When Ron was around, I couldn't help feeling a sort of affectionate amusement about his interactions with Hermione; they act like an old married couple. (As half of an old married couple, I can attest to that!) The way they alternately badger each other and comfort each other rang very true for me, so much so that I had no trouble believing that they were simply too busy staying alive (and keeping from killing each other in fits of temper) to think about sex. Plus I'd think the presence of Harry himself, living in close quarters as they were, would put a damper on any romantic interludes Ron and Hermione might otherwise have had. He's very like a brother to both of them, and a best friend to boot. I can't speak for everyone, but if I were on the run, desperately trying to save the world with no idea how to do it, and living in a small tent with my husband and our best friend, I'd feel awkward getting intimate with my husband, even if I COULD muster up the impulse. It would make the best friend feel horribly alone--who, in this case, is the one tapped to be the savior of the known world, so I'd be rather keen on keeping his morale up.
I will give you this, though: one relationship that absolutely did NOT ring true for me, and has not since its inception, is that between Harry and Ginny. That's not to say that it isn't a good match; it's a wonderful match, in theory. In practice, I feel that what few initimate scenes there have been between the two of them read as rather cool, and nothing in book 7 served to warm things up for me. It was sweet and all that Harry had a pang every time he looked at her during the battle of Hogwarts, but I didn't feel them with him. I feel like the entire romance has happened "off screen," as it were, and the only real evidence we ever see of it is Harry's occasional jealousy when a former boyfriend or other rival shows up. Harry's thoughts about Ginny herself, when he has them at all, always rang a little hollow and two-dimensional to me...and other than that little interrupted birthday tryst, which was itself a little stiff, (no pun intended), I never saw so much as a hint of actual sexual interest between them.
So I guess my conclusion is that while I enjoyed the book a lot more than you apprently did, and I don't feel it NEEDED a lot of sex per-se, I will agree with you on this point: Rowling simply isn't any good at writing realistic sexual relationships, and whatever sex the book COULD have used was notably absent.