Christopher Ryan at CNN:
Couples who turn to a therapist for guidance through the inevitable minefields of marriage are likely to receive the confusing message that long-term pair bonding comes naturally to our species, but marriage is still a lot of work.
Few mainstream therapists would contemplate trying to persuade a gay man or lesbian to “grow up, get real, and stop being gay.” But most insist that long-term sexual monogamy is “normal,” while the curiosity and novelty-seeking inherent in human sexuality are signs of pathology. Thus, couples are led to believe that waning sexual passion in enduring marriages or sexual interest in anyone but their partner portend a failed relationship, when in reality these things often signify nothing more than that we are Homo sapiens.
This is a problem because there is no reason to believe monogamy comes naturally to human beings. In fact, for millions of years, evolutionary forces have cultivated human libido to the point where ours is arguably the most sexual species on Earth.