It makes the story more interesting that it doesn't take the easy out by having Hario be an obviously BAD guy. But the more I read the more I'm certain these two were never suited for a romantic relationship. It's like Hario got his idea of romance from cheesy Hallmark movies: falling in love at first sight, never accepting rejection and continuously trying until she says yes, assuming her "yes" meant happily ever after and rushing into marriage. I'm also shocked at how little he understands her after 10 years. He's constantly surprised not just by the expressions his wife makes around Takara, but even mild disagreement from Emma surprises him.
Emma on the other hand seems to have a terrible home situation and saw Hario as the least bad option. She accepted him after getting worn down, but she's also committed to him because he's nice. Whether she realizes it or not, she sees him as nothing more than a friend she doesn't want to hurt. And as long as she didn't catch real feelings of love for someone, she was probably content staying together. (Though not without some change, as even before reuniting with Takara we learn Emma was insecure about her ability to be independent.)
The only reason I think these two have been married for so long, is because both have such agreeable personalities that there hasn't been any conflict through which they can justify calling their union into question, much less splitting up.
Also, I really hope he and Emma break up before the last volume, so we can explore Ema and Takara openly. It's better for both of them. Plus, it's a pet-peeve of mine when series wait until the last chapter for the main couple to get together. There's so much more to a romance to explore beyond the initial will-they-wont-they drama.