Unless we, as a society, can say why this is wrong, there is no logical reason to disallow this. I believe it is morally wrong, just as polygamy and homosexual marriage is wrong, but our society is so unwilling to say why that everything becomes permissible.
Hint:it isn't because these things are "icky" or different than the tradition.
An teenage revealed in an interview that she plans to marry her father and have children after dating for two years.