In my opinion I think that she ok with it cuz she still doesn't love him and even if they had sex but it's just that, sex. This was an arrange marriage for both parts that they couldn't refuse and although he was cheating on her they were lovers before she even appeared in the picture. I think its fresh that they exploring this relationships and not getting the same oh i married and now I'm supposed to be in love with you. With that being said I'm a really open person about a lot of stuff so ye ε=ε=(ノ≧∇≦)ノ
I agree with that just that I'm the type of person who gets annoyed by the genre of harem and I believe in monogamous relationships so I feel a bit iffy for it, I'm scared of reading more chapters because of their relationship but I can understand that not everyone is as close minded as me however I do believe that as long as both partners are okay with it then I'm okay with it too. Such a pity though I would probably be more into it if it was just the wife and husband or the husband and right-hand man.
Its an open relationship and while not everyone agrees with it, the wife has given the man consent and polygamy is a thing in that era, and I could understand that some readers are iffy about it because I am too. My heart aches in every moment he's with the right hand man cause I feel like he's cheating but he's technically not because the wife is okay with it. (I forget characters' names really easily so sorry if your confused on who I'm talking about.)