The thing is; I believe that Eri was the one getting married (shown in the first chapter) and they're just narrating what happened between the three of them in the past. If that's Eri, I'm fully convinced that she's not marrying Fuyuki (as Fuyuki wasn't seen in that timeline YET) and Eri probably met some other dude as she grew older, hence she decided to settle down. As we all know by now, Eri's the type to just go along with whoever (mostly dudes though, so Fuyuki's probably out of it unless we see some development).
My point is that regardless of what happened between the three of them in the past (especially between Fuyuki and Eri), it's not Fuyuki she's marrying in the present (which is likely). These are just representations of how complicated their relationship was.
Eri's the person who will most likely end up marrying a man than Fuyuki. If that's truly Eri, then we'll get to see what happened to her relationship with Fuyuki before she gets married.
If by any chance a miracle happens between the two of them and they both decide to pursue each other, then maybe there's a chance. But so far, we aren't seeing any of that.
