I swear, I've never seen a crowd of yuri fans so unhappy that a girl ended up in a happy relationship with another girl.
Literally any romance story ever is about "getting the girl" (or guy). You are virtually indoctrinated to expect the point of view character to get their love interest and to "only" regard this as a "good end". It doesn't matter that it might mean a "bad end" for secondary characters, whether the characters really make sense together, what other consequences there might be - only this matters.
I find the reaction unsurprising. ^^