Has it ever been estimated how many kids have different Fathers to those listed on the parents marriage certificate ?

Well, some studies estimate that in married couples, up to 1 in 5 of the kids, is not the offspring of the husband. That is one reason, I think every man should insist upon a paternity test, when a child is born.

A European study (small sample ) found that 10 to 15% are.

There were a couple of small scale studies, that showed almost half of the fathers were someone other than the name on the birth certificate. Truth is probably closer to 25% to 30%

Why does it matter?

I've never seen kids' names listed on someones marriage certificate.

Where do you live that children are listed on a marriage certificate?

What would the need to name children possibly be?

I have no clue, and don't really care.

nobody ever tried to find that, not important .......................