The final chapter of
Nazo no kanojo X. Yes, the manga has concluded.
Although the end brought about some sense of closure (unlike, for example,
Ranma 1/2), it wasn't what I had hoped it would be. After all, at the very start of the manga, Urabe tells Tsubaki that he will be her very first "sex partner"...
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...but we don't get to see this happen -- or get an indication that it will actually happen anytime soon.