In: Psychology
There are some social norms that do not depend on laws. For maintaining relationship the couple need to follow many rules and moralities that ensure the responsibilities, love and fellow-feeling. Marriage means cooperation and mutual understanding. Marriage ensures faith and mutual dependency. Both the partners are emotionally attached. Therefore there is sexual exclusivity, It is a social norm as well as personal preference and showing of steady emotional attachment.
Naturally it is unfair to ask one partner to another to sleep with third partner. It creates an emotional bonding between husband and wife. It is a holy relationship and family system depends on it. The growth of children also depends on the parental relationship and their attachment with the parents. Therefore it is a social system that the couple must be faithful to each other in doing sexual relationship. One partner should not ask for consent to sleep with third person. It disturbs family system. If one does so,he or she needs divorce and then they can do so.