Well, I actually feel like they were probably just 2 people who loved each other but had different needs. They're incompatible as lovers but they could probably stay friends as they care for each other. The biggest fault here is the guy didn't attempt to communicate his needs better and just hoped it would all work out.
It might have been the case that Midori would have been okay with things if they talked about it more. I know it can happen with couples where one is Ace but they're not possessive so they're okay with their partner seeing other people. In this case, Midori is more the low energy kind. And because Midori tolerated the cheating for years, it was probably more likely the lack of communication/understanding that made her fall out of love.
I don't feel bad for the dude but I understand Midori's behavior in the previous chapter more now.