But what is the exact math? Opinion ranges from -100 to 100. I'd think it's calculated by multiplying base fertility with mutual opinion, thus it might be something like this:
0 =100%
-100 = 0%
100 = 200%
Such odd design decision. They didn't bother reducing fertility with age, but still did this, despite there being several records of couples who hated each other having multiple children.
Again, I don't think negative opinion has negative effect. AI is designed to have 2 or 3 children on average. However, I think that positive opinion will raise that number slightly.
Not really sure if that is true though, dev would have to verify it.
EDIT:
There are these two defines:
BASE_NR_OF_CHILDREN_PER_COUPLE = 2,
EXTRA_NR_OF_CHILDREN_FOR_PLAYERS = 2,
However, I think these are the only opinion related things that might have impact:
NOT_SPOUSE_FERTILITY_MULT = 0.50, -- Applied to fertility when two lovers are not married
MARRIED_LOVERS_FERTILITY_MULT = 1.5, -- Applied to fertility when lovers are married
So yeah. Opinion has no effect on fertility, unless opinion is high enough to make two characters to become lovers.