Women get bored with their partners after about 1–3 years. They are less geared for monogamy than men are. This doesn’t mean you need to go poly (unless you want to) but it does mean that any long term relationship is going to need a lot of active work from both of you to keep it interesting and novel.

A strong libido and bored by monogamy: the truth about women and sex

“A couple live together, their libidos are matched, and they have a lot of sex. But after a year, two years, maybe three years, what tends to happen is that the woman’s desire drops more quickly than the man’s. At that point the woman thinks, ‘I don’t like sex any more.’ But what, in fact, is happening is that she is having a hard time with monogamy; because women get bored with one partner more quickly than men do.”