It is normal for relation ships to be monogamous
Mark feels he is being neglected by his wife.
Mark wants to have a hot sex life.
Mark tells his wife he loves her dearly and would like their sex life to be more exciting.
Mark talks to his wife about opening up their marriage to include affairs with other partners.
Mark's wife happily welcomes his ideas and feels their marriage is getting a second wind.
Mark's wife accepts, only because she is afraid he will leave her otherwise - but she is not happy about it.