
*spoilers*
How can you as an alpha date the person that organized the gang rape of your fated pair and destroyed your chance at happiness? And guilt is not enough, not enough at all.
No matter how happy our omega will be, that rapist guy of a seme still has a pair out there. I would have never never forgave them, not my fated pair for abandoning me, not those rapist and never that omega. Guilt is not enough at all.

Exactly. Not even a beating could be enough.
Right now, Rika is happy with that rapist seme. And he even said that he is good now because he has him and asked for him to be by his side forever.
Here comes the tricky part - what will happen when the fated pair of the alpha will come for him? Will the seme abandon his fated pair for Rika? I don't think so. It will mean one more life destroyed. Rika's happiness is temporary and he forgave everyone because the seme is by his side. What happens when he will be gone? Rika forgave too easily, way too easily. That omega destroyed at least 4 lives. A completely uninvolved omega will always be empty (the fated pair of the rapist seme).
I am somebody who is in a relationship for years. Trust me when I say that butterflies can be rekindled. They may disappear at some point, but they can be resurrect if both partners have a good communication. If the chemistry was really good at the start of the relationship, then butterflies can come again. Don't worry and give up.
Sinwoon's biggest mistake is that he started to take Yoonsung for granted. He started to assume that he will always be there. This is not how it works. Both partners must think of the other's feelings in order to make it work and keep the relationship strong. A relationship is something you maintain. There may be ups and down, but as long as the partners keep being on the track, they can be happily together.