@boag
There is one story where a dude gets isekaid and married royalty, it is sort of a politically focused slice of life romance, more or less.
I would guess it is the opposite of what you said though, because in that one, the mc refuses to have a concubine or second wife, although, the other nobles keep being pushy about it.
This story gives me so much anxiety like I always hold my breath while reading. I love the female main character. I feel so bad for her situation but she pulls through every obstacle with dignity and grace