Lol of course it can be considered cheating. It's not about law but about your feelings and being loyal to the one you love. He can have many concubines but still choose not to get involved with any of them and that would be so cool, but instead he's just using him and has sex with him to make Yuer jealous. And that's disappointing and I'd love not to see it in the manhwa
I know all that
But try to see it from the point of them
This isnt a modern world its historical
Idk if you've read these type of things or not but there are love stroy which they are in love but he has to sleep with someone else so he can have an heir
Or when they are same sex couple but because no one can know one of them marry someone as his bear
These are just some of it
If we are looking at it realistically normally empowers could be in love with some one yet spend a night with some one easily
What I'm trying to say is this things happen,they can be love....it wont give u the perfect love story but it has it's own values
Although there is a great difference,in the ones I mention the other party understand the decision and has no problem with it ( its consensual) and it's not "I was feeling mad so I was with someone bare with it " kind of things
I hope you get what I mean
Btw the seme is trashhhh dude can get fucked for all i care

SPOILER(?)
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I just hope they change the story a little in the manga and don't follow the novel because what I hate the most is cheating and if he really goes and has sex with the other concubine to make him jealous then I'm so done with it.. at least in the manga, don't make me go through it once again.....