This from BBC news:
A woman’s sex drive begins to plummet once she is in a secure relationship, according to research.
Researchers from Germany found that four years into a relationship, less than half of 30-year-old women wanted regular sex.
“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.