
Emotional cheating is a thing and possibly worse than physical because at least if it was just physical it was only lust and no feelings were involved. With emotional to a degree they have feelings for someone else. It's not just about acting on them. Just the fact she was (and she admitted she was) catching feelings for someone else shows they needed to take a step back. The breakup was a good thing.

Emotional cheating is definitely real and a big time relationship killer. If your guy or gal is out fantasizing and blushing over a friend or coworker you've got some trouble on your hands.

LoL
Have you ever been in a relationship?
Because you love someone doesn't change the fact that you still can be attracted to other people. Wtf. I'm happily married for 8 yrs now and I'm telling you that I still drool over hot guys. I'm sure my husband can still appreciate nice tits on other girls and it doesn't change the fact that neither of us will cheat. Grow up.

Drooling over other people is not emotional cheating. Have you actually read this manga? It looks like you didnt since what you describe is totally different from what happens. Mio developed feelings for Marui. This is why she apologizes to Taki and basically tells him she was unfaithful. She was in love with both of them but kept denying it until things got messed up. This is also why Marui did not give up so easily as he had planned. He noticed it too when she got jealous over him interacting with another girl.

"attraction" doesn't equal "emotional attachment". If you've been married that long and can't tell the difference, there are other issues you should really work on.
Overly jealous and possessive partners will [undeserverdly] kick up a fuss when they learn of their partners' attraction for someone else.
But if said partner were to be longing for someone else, or would compare their relationship with how it would be with that third party, that's a clear case that their heart no longer belongs to their partner, and that they have already cheated on them, even if they haven't had that kind of physical contact.
I wasn't trying to get insulting or anything, but I was a bit ticked off with your reaction that seemed to imply you were mature because you couldn't acknowledge that there was more to a relationship than just sex :P
Some really overreact. Taki was the one who misunderstood and she clearly didn't cheat on him, everyone can have a moment of weakness as long as they don't act upon it it's fine.