Entropy: a process, or the measurement, that shows the tendency of things to go to a state of disorder over time.

Heat Death: The inevitable end of everything. The finite supply of energy spreads thinner and thinner throughout the expanding universe until there is an infinitesimal amount (none) at any one place.

Relational Entropy: the tendency of relationships to go to shit given enough time.

Relationship Heat Death: the inevitable end of a relationship.

Without additional energy put into the health of the relationship it will decay. This should be obvious to anybody who has been in a relationship for more than a month or two. The interesting things to consider is why this is required, and the means by which men and women add energy to the closed system of a relationship.

Women use sex to get love. Men use love to get sex. Put another way: sex is a means to a woman's desired end, while commitment is a means to a man's desired end. This is the dance new couples go through. This is why fresh relationships are so exciting. Both sides are using their tools (sex for women, commitment for men) to maximize what they get from a relationship. This balance works at first, but as time goes by the man's tool becomes weaker and weaker. As he gives his woman more and more commitment he has less and less to give. Once he marries her he no longer has his tool for gaining his desires. The man no longer has any means to add energy to the system. Without the incentive of increased commitment there is no purpose for a women to add any more energy than is absolutely required. Just like many people only work hard enough to not get fired, wives give the bare minimum amount of sex. Marriage is the state where men cannot add additional energy and women have no incentive to do so.

A man's tool for adding energy into the relationship is finite. He eventually runs out of commitment to add. Relationship Heat Death is inevitable.