Why women in a long term relationship are less interested in sex?

Researchers from Germany found that four years into a relationship, less than half of 30-year-old women wanted regular sex. Conversely, the team found a man's libido remained the same regardless of how long he had been in a relationship. Writing in the journal Human Nature, the scientists said the differences resulted from how humans had evolved. For men, a good reason their sexual motivation to remain constant would be to guard against being cuckolded by another male

Dr Dietrich Klusmann, lead author of the study and a psychologist from Hamburg-Eppendorf University, believed the differences were down to human evolution. He said: "For men, a good reason their sexual motivation to remain constant would be to guard against being cuckolded by another male." But women, he said, have evolved to have a high sex drive when they are initially in a relationship in order to form a "pair bond" with their partner.
But, once this bond is sealed a woman's sexual appetite declines, he added. He said animal behaviour studies suggest this could be because females may be diverting their sexual interest towards other men, in order to secure the best combinations of genetic material for their offspring. Or, he said, this could be because limiting sex may boost their partner's interest in it.