This setting seems to be set
under several situations. If characters get Engaged then all other
character relationships will have this setting, If the age groups differ
such that having children isn't an option then this setting is set between
two characters, And it is also set between family members to prevent them
from inappropriate actions. I.E. Removing it between Teen Siblings is
likely to enable them to flirt with each other which would be
inappropriate between family members.