To clarify, other estimates put misattributed paternity in the range of 1-4%, but that's still a whole lot of marriages (especially considering that most marriages have multiple kids).
While that may be true, that is not what happened in this case. I also thought that was where the story was going based on the title, but it's not where it ended up. The author found a half-brother: before the author's father married his mother, his father put up a baby for adoption with a former girlfriend.