If you want to keep your long term girlfriend interested in sex, don’t marry her, and keep her slightly on edge about the possibility of losing you to the competition.
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.
