Franky knows what it’s like to be abandoned by a father and the loneliness that results . But he also knows what it’s like to be taken in by a father figure. And now, he doesn’t want others to feel what he did as a child. So he pays what Tom did for him forward—while he’s around, no kid will be fatherless!
Because, as I recently discovered, this sort of thing is even more intimate in Japanese culture than it is in Western, and it’s only really done between couples. Even in this SBS, Oda refers to this bit in the context of a married mother and father.