Calm down, hold your horses, this is still about writing.
So I really got the urge to make my MC had s*x with one of the side character. But solely for the reason of realism since life on medieval fantasy can be so boring and both of them are adults and had taste s*x before.
But I dont want to make them end up on having a relationship at all and keeping it casual and professional instead after all the haba-haba, because, well... that's what happen irl :|
Will this makes my readers angry if im doing this without the intention of making them having a relationship?
And when that happen again in the future with another characters, will it makes my story considered as a Harem?
PS:
1. I'm not writing a smut or its cousins.
2. If your environment and beliefs frown at this kind of things, then I respect you. But please give respect also and don't get triggered since this is normal in some places.
It's your story. Do it if you want to. You don't need to justify yourself to anyone about your creative decisions within your own creative works.
It's not something that I'd do regularly though, but if it fits within the narrative, and both characters personalities, then you should do whatever you want to do.
If it doesn't, and you still want to do it, then change the characters, and narrative, so it does.
As I said before, it's your story. Write what you want.
I'm actually doing something similar for the Bonus chapters in Duskfall.
They follow the character, Sylvia, and her
antics. She isn't one to settle down. At all, and the idea of monogamy is completely alien to her.
All of that, however, fits quite nicely within her characterization without breaking the world, or the characters. It's not narratively relevant, hence why it's "bonus content".
Though, if you're aiming for realism in the Medieval Fantasy aesthetic, there's plenty of other aspects of life within the setup that aren't "boring". You could definitely look into the historical activities of the people during the era to find plenty of things to let the characters occupy their time beyond just sex.