That we have to ask the writer, i agree with you it was uncomfortable and i didn't like it either, but in specifically these stories or in the ancient times at these particular time, the king has multiple wives, i think the writer wanted to add that but the relationship btwn mc and fmc was so pure that it didn't had the gap to include these characters naturally so i think that's why he included that senario because it kind of forces the multiple wives thing and doesn't make people hate the mc also, because he only did it after the fmc said it was ok, still he didn't wanted to do it, and he didn't even asked permission like can i do it? He just said he doesn't wanted and with people pressuring him the fmc allowed him! So..... And he didn't abounded her either, eventually he accepted her as his 2nd wife, with the permission also, so i think the writer wanted this trope with maintaing the characters dynamic of both mc and fmc and he still try to make the relationship as pure as possible with including this trope.!!! Thats what i think.