I think that has to do with the way schools work in Japan. Two people going to the same high school might live on completely opposite ends of town. Once one graduates, they won't necessarily be "in the neighborhood" like you might have in the American school system.
D.Va's kind of an outsider in the cast. She's not really a member of Overwatch, is she? And she's not a villain. I think it's the fact that she's cute and doesn't really have a background with any of the other characters that makes it easy to ship her with pretty much everyone.
It was too much for her, loI. I kind of get the impression she could match her girlfriend in being a pervert, she just needs to work on her inner barriers~