Even worse, they get married, have children, and THEN find out that they are half-siblings.
Which is very, very possible. After all, how do a lot of people find doctors and lawyers and other professionals? They ask their friends. So let's say a woman is having fertility problems, she knows that her friend Sue in her book club was treated for infertility and is now pregnant, she asks Sue who her doctor was, goes to him, bingo! Pregnant - but she doesn't know her baby's father is also the father of Sue's baby.
Multiply that scenario by a lot of women who know each other, whose kids go to the same schools or socialize together, grow up, date each other, marry...
Yikes.