People normally differ in the degree of sexual appetite they have. There is no single standard of sexual desire, and desire differs not only from person to person but also for the same person over one's lifespan.
One of the most common sexual complaints among couples is a disparity in sexual desire. Sexual desire can be low for a variety of reasons, many of them psychological and interpersonal. But that doesn't necessarily make it a disorder. It becomes a diagnosable condition only when it diminishes the quality of one's life and creates distress, or a disparity arises in the s*x drives of partners, evolving into a matter of unresolved contention in the relationship. Loss of sexual desire can both result from relationship problems and cause them.
Moreover, what constitutes "low" (or hypoactive) sexual desire is relative. Partners who use the degree of sexual desire experienced early in a relationship as a standard of comparison may label as a problem the drop in sexual desire and activity that often accompanies long-term partnerships. Further, a person who experiences low sexual desire that is problematic relative to one partner may not experience any disparity in desire with a different partner. What is designated as one partner's low level of desire may more accurately reflect a hyperactive s*x drive in the other partner.
S*xual desire and responsiveness normally differ between men and women, and assumptions about sexual equivalency may falsely suggest the existence of hypoactive desire disorder. Men are more readily physiologically aroused than women, and, for them, desire is tied tightly to this arousal. Among women, s*xual desire is typically more psychological and situational, influenced by how they feel about their bodies as well as the quality of relationship with their partner. Moreover, women often do not experience desire until after they are genitally aroused, and arousal may require an extended period of foreplay.
The waning of s*xual desire is sometimes considered inevitable in a long-term relationship, but it is unclear whether that is truly the case or whether it is a function of age or familiarity. Low sexual desire can often be treated. Increasingly, experts are optimistic that the sexual spark can stay alive throughout the lifespan.