Responses

It is because back then marriages were a transaction, and people rarely married for love (I think?) so since the child was made out of the marriage that wasn't love = the kid is a love child ! Because the parent must have went to find love somewhere else. That's my interpretation.
Today, with a lot of marriages made out of love, it is surely bound to create some confusion ! But back then I guess it made sense.
I have serious doubts that Finne is a love child. My guess is that MC's father abused his power against the poor maid. (ノꐦ ⊙曲ఠ)ノ彡┻━┻
where is mc's biological mother?