"In reality the mother could have married someone who could have been the father."
The mother may be married. We don't know since the story never discussed the teenagers background other than to indicate their birth was due to artificial insemination.
Why do you keep insisting the mother aren't married and the kids don't have fathers who have raised them?
Why are the kids "longing" to find their biological fathers?
Because many of these mothers do this sort of thing.