If
a man gives his wife a Get on condition that she marries so-and-so, she
should not marry so-and-so, because it looks like giving wives as
gifts. If she does marry, she need not divorce. She should not marry
others, because this may invalidate the Get.
"On condition that
you have relations with so-and-so" is valid, because she can bribe him
to marry her for a time. "On condition that you have relations with my
father, or your father" is not valid, because she can not bribe the
father, so it is to vex her and the Get is valid.