You're talking about "Happy End"? Cause if not I'm curious about what manga you're talking about.
Yep. That's it. That's the one. There it is.
...
Man, fuck Happy End.
And I've thought about it again a bit more.
While this one is entirely open-ended, there's not much reason to say that they necessarily didn't get together, and even less reason to say that their relationship was any more than probably temporarily strained.
Think about it for a second. There's no end on the manga. Matsuri didn't pick up the boy's letter again (or at least it wasn't shown) and at no point did it suggest she thought about the boy again. Matsuri is evasive from Koyuki and tends to blush heavily when they do actually interact. Matsuri doesn't seem to be rejecting or denying Koyuki and, unless wham sudden curveball additional part and character development with no previous hints, she doesn't seem to have any sort of condition or anxiety that would actually cause her to be (nearly) unable to tell Koyuki that she's not interested. This is a yuri publication, filled with yuri girls and yuri in general, to be sold to an audience that likes yuri and probably doesn't like getting teased and then heavily rejected/denied of yuri.
While it could theoretically end up not being yuri, I think it'd be a bit silly to consider it such, and it'd be even sillier (or maybe just stupid) if it really did end as something other than yuri.