I played through the game when it first released and i guess i got a different impression of what happened. The second part of the game after the mc and heroine got married was mostly netorase in my eyes with a side of netorare. During the scene with the king, i just saw it as both the mc and Sera getting really into the netorase and less that she was really betraying him. After that scene, the rest of the game is pretty much just more netorase with the mc and Sera reaffirming their love through their fetishes. Even the extra scenes that were added a couple of months after the game's release were showing how much more Sera enjoys other men's sex over the mc but that she'll always love him. I saw it more as Sera being very slutty in part 1 and then after she gets married to the mc she pretty much restrains her desires. Once the mc reveals his ntr fetish to her, she also takes this as an opportunity to let her desires loose. When the mc finds out about her desires, just like she accepted him for his fetish, he accepts her libido and so they love each other even more and develop more trust towards each other.
I think this is especially well demonstrated in Suzuya's latest ntr game which had most of his characters from previous games swap wives. There's a particular section of the game where Sera and Chloe (another heroine from a different game) have to serve a random rich dude in another world. Ex and the mc of that game decide to go peep on them only to find that both girls who are well known for their near endless libidos were both thoroughly fucked. Its Sera's best time having sex by far and the rich guy even offers them to stay with him as concubines however Sera instead decides to stay with her husband. By the end of the game, Sera and Ex go back to their own world which means that Sera would never be able to see the rich dude again so its pretty safe to say that didn't completely fall to anyone. To go a bit further, the world that they get transported to is literally a world where sex is way more commonplace and is even the source of their magic power and there's likely a lot more guys on the level of the rich dude if not even higher than that so if there was any time for Sera to ditch Ex, it would have been that time.
So yeah, that's my take on Sera and the MC's relationship. Seems i got a bit passionate on the subject. Tbh, when playing through the game, i was hoping for a proper ntr ending or at least one that's more hardcore but at the end of the day i still loved the direction that the game went. I want to play the translated version to see if i missed anything but i'm currently away from my pc so no ntr games for quite some time for me. Do tell if i got anything wrong.