https://www.mangago.zone/read-manga/kataomoi_to_parade/mf/sa-v2c010/pg-18/
Is it this one? It sounds incredibly similar to what you've said.
I don't remember the title but it seem like I could give you another hint if we think the same.
They are most like a childhood friends. It is the seme who introduce the uke to his father with the POV " I have a two sons now. I guess I can't see my my grandaughter or grandson in the future. But as long as both of you are happy I'm fine with it."
https://www.mangago.zone/read-manga/kataomoi_to_parade/mf/sa-v2c010/pg-17/
Sorry it's rather this one
its either the uke or seme have introduced their boyfriend to his father and he accepted them wholeheartedly and even said "I have two sons now"
I think the father had somehow a hint of their relationship even before they told him
Stories have been mixed up in my head at this moment and my list and notes are not helpful (/TДT)/ please help me
its not
https://www.mangago.zone/read-manga/rational_pervert_romance/
or
https://www.mangago.zone/read-manga/subarashii_kiseki_ni_yasashii_kimi_to/